86LX 5.0 pops,sputters,backfires

[EliteLX]

Hi all, new to the site but long time browser. Been building and working on Foxbody Mustang's for over 15yrs in my spare time as a hobby. I broke a torque box last fall and the car sat all winter with very little gas in it. Once I fixed the box and took the car for a drive I noticed something very odd. When the car is ice cold everything is fine. After about 3 minutes it goes real bad. It bucks and spits and pops all the way through the rpm band and when you let off the throttle, huge backfires occur out the tailpipes. I thought this was just an under load problem but if you rev it in neutral around 4500rpm it starts to break up all the way to 5500. Trying to go WOT in any gear just doesn't feel or sound very good with the sputtering,backfiring,bogging,etc. I have spent a lot of time researching this problem and have troubleshot it to death. Have had the motor down to the heads twice so far checking everything mechanical,electrical and in between. Things i have replaced or checked...wires,plugs,cap,rotor,TFI coil,checked for vacuum leaks,timing,fuel pressure,lifters,rockers,new pushrods,EEC is good, checked for frayed wires. I did KOEO and KOER tests with a test light and paperclip. I have removed the smog pump and AC so most of the codes had to do with that.

KOEO CODES 81,82,85
KOER Codes- 94,44,33,88

Again, for the first 2 minutes car runs like a dream. I try not to get on it as we all know mustang's like to be warmed up first. After that it bucks,spits,sputters,backfires,and bogs. At idle you can hear the engine stutter if you stand near the exhaust. It does this every few seconds then returns to normal idle.My mods are as follows...

1986 MustangLX 302 5spd. Speed Density car, NO MAF
Trick Flow street/strip intake-BBK Adj fuel press regulator-BBK 255lph Fuel Pump-BBK shorty headers-2 1/2" free flow cats-H-pipe to SLP resonators-1.7 crane cams roller rockers-ford racing pushrods-CAI with enormous cone filter-removed smog&A/C running short belt-3.73 gears-short throw shifter-Hotchkis suspension-3 core radiator. stock heads,stock cam,stock block.

I've had these mods for years and the car has always ran like a champ.
My personal conclusion is the fuel return line may be clogged up forcing the fuel pump to overheat after a couple of minutes. The problem with that theory is my fuel pressure gauge holds a steady 42lbs at WOT under load even when its breaking up badly( have an external mounted fuel pressure gauge). Would love to hear anyones theories. Nearly stumped and running out of options. Just ordered a distributor today which is the last piece of the ignition system making it completely new and putting me about $900 into the troubleshoot/repair.

 

[Kdubslugga]

So even at idle when it gets warmer it will start run rough, or its only bad during a heavier load? Sounds like a fun one! Have u checked the timing on the car? I would verify that just to start ruling things out. Youve done plugs and wires which is where I would have said to look next. Id rule out a vacuum leak because it wouldnt matter if the engine was cold or not with that. Is it possible your EGR us sticking?

 

[Kdubslugga]

If Im not mistaken a code 81 is a Map sensor code.

 

[EliteLX]

A curios thing about the EGR. When I hook a vacuum hose to it, everytime i go to release the clutch the car wants to stall. If i remove vacuum hose it doesn't do it....Timing is good, car idles fine even when its warmed up. Anything over 3k rpm starts to break up badly.

 

[jrichker]

KOEO CODES 81,82,85
KOER Codes- 94,44,33,88

The 88 code isn't valid for 5.0 Mustangs. All the other codes listed are emissions equipment related. You car description suggests that all the emissions equipment has beeen removed. In that case, none of the codes would point to anything that could be the source of the problem.

Some basic theory to clarify how things work is in order…

EGR System theory and testing

The EGR shuts off at Wide Open Throttle (WOT), so it has minimal effect on performance. The addition of exhaust gas drops combustion temperature, increases gas mileage and reduces the tendency of the engine to ping. It can also reduce HC emissions by reducing fuel consumption. The primary result of EGR usage is a reduction in NOx emissions.

The EGR system has a vacuum source (line from the intake manifold) that goes to the EVR, computer operated electronic vacuum regulator. The EVR is located on the back of the passenger side shock strut tower. The computer uses RPM, Load. and some other factors to tell the EVR to pass vacuum to open the EGR valve. The EGR valve and the passages in the heads and intake manifold route exhaust gas to the EGR spacer (throttle body spacer). The EGR sensor tells the computer how far the EGR valve is open. Then computer adjusts the signal sent to the EVR to hold, increase or decrease the vacuum. The computer adds spark advance to compensate for the recirculated gases and the slower rate they burn at.

Troubleshooting:
There should be no vacuum at the EGR valve when at idle. If there is, the EVR (electronic vacuum regulator) mounted on the backside of the passenger side wheelwell is suspect. Check the vacuum line plumbing to make sure the previous owner didn’t cross the vacuum lines.

Diagram courtesy of Tmoss & Stang&2birds. (the diagram says 88 GT, but the EGR part is the same for 86-93 Mustangs)

The EGR sensor is basically a variable resistor, like the volume control on a radio. One end is 5 volt VREF power from the computer (red/orange wire). One end is computer signal ground (black/white), and the middle wire (brown/lt green) is the signal output from the EGR sensor. It is designed to always have some small voltage output from it anytime the ignition switch is the Run position. That way the computer knows the sensor & the wiring is OK. No voltage on computer pin 27 (brown/lt green wire) and the computer thinks the sensor is bad or the wire is broken and sets code 31. The voltage output can range from approximately .6-.85 volt.

The EVR regulates vacuum to the EGR valve to maintain the correct amount of vacuum. The solenoid coil should measure 20-70 Ohms resistance. The regulator has a vacuum feed on the bottom which draws from the intake manifold. The other vacuum line is regulated vacuum going to the EGR valve. One side of the EVR electrical circuit is +12 volts anytime the ignition switch is in the run position. The other side of the electrical circuit is the ground path and is controlled by the computer. The computer switches the ground on and off to control the regulator solenoid.



EGR test procedure courtesy of cjones

EGR test procedure courtesy of cjones

to check the EGR valve:
bring the engine to normal temp.

connect a vacuum pump to the EGR Valve or see the EGR test jig drawing below. Connnect the test jig or to directly to manifold vacuum.

Do not connect the EGR test jig to the EVR (Electronic Vacuum Regulator).

apply 5in vacuum to the valve. Using the test jig, use your finger to vary the vacuum

if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.

if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.

if engine stumbled, connect EGR test jig to the hose coming off of the EGR Valve.
Use your finger to cap the open port on the vacuum tee.
snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
did the vacuum gauge show about 2-5 in vacuum?
if not the EVR has failed

EGR test jig

To test the computer and wiring to the computer, you can use a test light across the EVR wiring connectors and dump the codes. When you dump the codes, the computer does a self test that toggles every relay/actuator/solenoid on and off. When this happens, the test light will flicker. If the test light remains on the computer or the wiring is suspect.

To check the EVR to computer wiring, disconnect the EVR connector and connect one end of the Ohmmeter to the dark green wire EVR wiring. Remove the passenger side kick panel and use a 10 MM socket to remove the computer connector from the computer. Set the Ohmmeter to high range and connect the other ohmmeter lead to ground. You should see an infinite open circuit indication or a reading greater than 1 Meg Ohm. If you see less than 200 Ohms, the dark green wire has shorted to ground somewhere.

Late Model Restoration may still have the Ford Racing M-12071-N302 kit with the EGR valve & sensor along with the ACT & ECT sensors for $45. See http://www.latemodelrestoration.com/iwwida.pvx?;item?item_no=M12071N302 1&comp=LRS for more details

 

[EliteLX]

I do appreciate the info on the egr system and how it works which i already knew but, it has zero to do with the problem at hand. I could just put an egr delete plate on there and be done with it but, the car would still spit and sputter at WOT. I am in search of a solution to the problem. Please refrain from copy/pasting out of online manuals. I'm looking for someone who had a similar problem and how they may have fixed it.I have been working on and building mustangs for close to 20 years. I know EFI stangs like the back of my hand but, I have never had this type of problem before. Thanks

 

[jrichker]

I do appreciate the info on the egr system and how it works which i already knew but, it has zero to do with the problem at hand. I could just put an egr delete plate on there and be done with it but, the car would still spit and sputter at WOT. I am in search of a solution to the problem. Please refrain from copy/pasting out of online manuals. I'm looking for someone who had a similar problem and how they may have fixed it.I have been working on and building mustangs for close to 20 years. I know EFI stangs like the back of my hand but, I have never had this type of problem before. Thanks

The EGR tech note isn't a copy and paste of some online manual. It is the result of my personal study and efforts to present the EGR system to people in an understandable manner. There is very little information except for some diagrams that is copied or borrowed in any of my tech notes. I try to give credit when I do use something that is copied or borrowed.

The manner in which you described connecting the EGR to vacuum left the impression that you bypassed the EVR (Electronic Vacuum Regulator). That would have cause the engine to sputter and/or stall.

According to your description, the only thing left that you haven't checked or replaced is the PIP in the distributor.

PIP problems & diagnostic info
The typical failure mode for PIP sensors is that they start to fail as the engine gets warmer. On a cold engine , the PIP may not exhibit any problem symptoms.

Spark with the SPOUT out, but not with the SPOUT in suggests a PIP problem. The PIP signal level needs to be above 6.5 volts to trigger the computer, but only needs to be 5.75 volts to trigger the TFI module. Hence with a weak PIP signal, you could get spark but no injector pulse. You will need an oscilloscope or graphing DVM to measure the output voltage since it is not a straight DC voltage.

See http://www.wellsmfgcorp.com/pdf/counterp_v8_i2_2004.pdf and http://www.wellsmfgcorp.com/pdf/counterp_v8_i3_2004.pdf for verification of this little detail from Wells, a manufacturer of TFI modules and ignition system products.

 

[EliteLX]

Now that sounds like something that could have been the problem which is why i installed a brand new distributor 3 days ago. Still having the same issue. When i run cylinder test it constantly gives different codes. I.E. first time i got code 8, i moved sparkplug in cyl 8 to cyl 7 then reran test. It gave me a 7,2,and 1 as being problematic. I have run the cylinder test many times and each time it gives different cylinders as problematic. Have brand new plugs and wires, new distributor, proper fuel pressure to the rail,brand new MSD TPI, new cap and rotor, new ford racing pushrods,have switched out EEC for a spare and still no change after the car warms up. There is a huge difference when it is cold as it idles much better and runs very strong which is expected in a car that has had $1200 worth of brand new parts. As it warms up, it gets progressively worse from spitting and sputtering at 5k to 4k then finally all the way down to 3k which is the cutoff. This happens even if I'm on partial throttle anything over 3k rpm after the car has come to full operating temps and its spit, sputter and backfire city. I apologize for the egr rant. I'm just really fed up with this issue and spring is here. I want to be at the track and this problem is killing my season so far. I have this posted on many Mustang forums and noone has a clue. I'm getting close to going carbureted and being done with it.

 

[7upstang91]

Very interesting. I assume you have fresh gas as well.

To be honest, it sounds like a fuel problem to me. I bet you have tried this already and haven't listed it, fuel filter? What do the new plugs look like?

I know I probably am not adding any knowledge, but I am trying to help trouble shoot.

What about the MAP sensor? I have read many things about a bad MAP causing bad fires.

 

[jrichker]

The wandering results of the cylinder balance test are due to the fact that the computer cannot control the engine RPM. Usually it is because it cannot hold a steady 1450-1650 RPM.

Since you have swapped most of the replaceable parts, it is beginning to look like a wiring problem. You will need a DVM to check the resistance of the wiring between the TFI and computer. Wiggling the wires and connections as you test them is a good plan. A hair dryer can help supply the heat to bring things up to operating temperature.


What I have done is to use a 3 ohm 100 watt resistor or a light that draws 2-5 amps worth of current and measured the voltage drop across the computer wiring.

The process goes like this:
1.) Disconnect the computer connector from the computer.
2.) Pick a circuit to check, like the TPS signal, which uses pin 47. The computer wiring harness connector uses a female socket. Use a nail, or cotter pin along with an alligator test clip to ground pin 47 to the ground wire near the computer. I took a 1” piece of solid copper wire and chucked it an electric drill to file it down to the correct size. Then I soldered it to my ground test wire. You may have some pins with the wires already crimped on that you could use to make a ground test wire.
3.) Disconnect the other end of the circuit and connect a 2-5 amp test lamp (or 3 ohm, 100 watt resistor) to the circuit with the other side of the test lamp or resistor connected to the battery.
4.) Connect one test lead from the digital voltmeter (DVM) to the ground point inside the car. Connect the other lead of the DVM to the junction where you connected the test lamp or resistor to the circuit under test, for example, the green wire on the TPS.
5. With the light glowing, measure the voltage across the wire. This will be the lost voltage due to the resistance of the circuit. You should see less that .5 volt. This is were the hair dryer and wiggle test comes into play.


Diagram courtesy of Tmoss & Stang&2birds

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

 

[EliteLX]

Thanks man, I will get on it this weekend.

 

[EliteLX]

Very interesting. I assume you have fresh gas as well.

To be honest, it sounds like a fuel problem to me. I bet you have tried this already and haven't listed it, fuel filter? What do the new plugs look like?

I know I probably am not adding any knowledge, but I am trying to help trouble shoot.

What about the MAP sensor? I have read many things about a bad MAP causing bad fires.

Changed fuel filter, new plugs but i just bought another set that I am going to try just in case, getting good fuel pressure to the rail(have external mount fuel pressure gauge), no clogs in fuel return line. Plugs don't look lean or rich. When I pull them, they look pretty much brand new. Never thought about the MAP sensor and after reading what it does that may be the problem as well. Something else to throw money at woohoo!

 

[mark5]

Hey everyone...I stumbled across this post a couple min ago and am interested in it because I have a 94 Mustang GT with a very similar problem, except mine backfires out the exhaust at idle(and it doesn't matter if its cold or warmed up) along with the breaking up above 3k rpms. I've replaced everything ignition wise and ran multiple test. So I'll be watching this thread pretty close to see what solves EliteLX's problem...I'm in the process of checking wires now so I'll try the process described above. If I find an answer before EliteLX does I'll post what I found, if not I'll keep watching and testing.

 

[EliteLX]

Ok, so today I replaced all the platinum plugs with regular autolite replacement plugs, no fix(platinum 3 week old plugs looked almost brand new). Unplugged MAP sensor still ran the same. Pulled each plug wire individually with the car running and each stuttered as it should when the plug wire is pulled. Electrocuted myself 3 friggin times so I can attest that the wires are getting PLENTY OF JUICE! Played with timing and fuel pressure up and down the scales with no sweet music at the end. Looks like I am going to have to look into the nightmare that is my wiring. Here is a video of the car as of today just so you guys can hear what I'm talking about. Sorry for the crappy cellphone quality...

YouTube - mustang breakin up bad

 

[mark5]

Ok, so today I replaced all the platinum plugs with regular autolite replacement plugs, no fix(platinum 3 week old plugs looked almost brand new). Unplugged MAP sensor still ran the same. Pulled each plug wire individually with the car running and each stuttered as it should when the plug wire is pulled. Electrocuted myself 3 friggin times so I can attest that the wires are getting PLENTY OF JUICE! Played with timing and fuel pressure up and down the scales with no sweet music at the end. Looks like I am going to have to look into the nightmare that is my wiring. Here is a video of the car as of today just so you guys can hear what I'm talking about. Sorry for the crappy cellphone quality...

YouTube - mustang breakin up bad

Man I FEEL ur pain. My car is doing the exact same thing. When I figure out how to do it I'll post the youtube vid of it from 2-3yrs ago. I know, I know
, that's a long time. I just didn't want to jump into the wiring harness, but if I want to race this yr I have to get it figured out.

 

[mark5]

Man I FEEL ur pain. My car is doing the exact same thing. When I figure out how to do it I'll post the youtube vid of it from 2-3yrs ago. I know, I know
, that's a long time. I just didn't want to jump into the wiring harness, but if I want to race this yr I have to get it figured out.

Here's the vid, u can't hear it breaking up , but it is. Also u can't hear the turbo which u should. I heard the turbo on the street when I was trying to figure things out. Anyway here u go.

http://youtu.be/k538Jc4c5Lw

 

[7upstang91]

I would say, the MAP or fuel injectors. I don't know how expensive the MAP would be, but you can always return it.

 

[EliteLX]

I would say, the MAP or fuel injectors. I don't know how expensive the MAP would be, but you can always return it.

When I unplugged the map sensor (which defaults its abilities) it still ran terrible and idled beyond terrible. Injectors is my next "to buy" item. Plan on getting the Generation3 injectors with the old style plug. They are suppose to be alot better than stock. They have 4 holes that spray fuel as apposed to 1. Doubt it is an injector mainly because it runs really strong when it's cold. If an injector was bad, think it would act accordingly all the time, not just when the car is warmed up. BTW Mark, Holy crap! With the setup on your car that could be a number of problems but i bet on 5cylinders it's faster than mine on a good day, sheesh!

 

[7upstang91]

When I unplugged the map sensor (which defaults its abilities) it still ran terrible and idled beyond terrible. Injectors is my next "to buy" item. Plan on getting the Generation3 injectors with the old style plug. They are suppose to be alot better than stock. They have 4 holes that spray fuel as apposed to 1. Doubt it is an injector mainly because it runs really strong when it's cold. If an injector was bad, think it would act accordingly all the time, not just when the car is warmed up. BTW Mark, Holy crap! With the setup on your car that could be a number of problems but i bet on 5cylinders it's faster than mine on a good day, sheesh!

I have no idea. I am out of ideas. I remember in my old coupe, if the gas got under 1/4 tank it would stutter and hesitate just like you describe. I never got around to replace pump, just throwing another idea out there to help.

I am more so trying to pick your brain, trying to help. Good luck man, sorry I can't help much.

 

[EliteLX]

Learned today that bad/clogged catalytic converters can also cause this problem. The hotter the cats get the more they clog and can make the car stutter, spit and backfire. Exhausted today but tomorrow am going to sawzall the cats out and run it for awhile.