Car won't hold idle

There is no easy way to find vacuum leaks. It is a time consuming job that requires close inspection of each and very hose and connection.

Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-7250 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.

Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

The vacuum line plumbing is old and brittle on many of these cars, so replacing the lines with new hose is a good plan. The common 1/8” and ¼” vacuum hose works well and isn’t expensive.

Diagram courtesy of Tmoss & Stang&2birds



Vacuum leak due to slipped lower intake manifold gasket...

Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. Spay everything with anything you have, and you won't find the leak...






Once you have eliminated any vacuum leaks, here is the next place to check out...

See the "Surging Idle Checklist for help with all your idle/stall problems.

The quick and easy way to dump the codes is in there too, and all you need to do it is a paper clip! The first two posts contain all the updates to the fixes. At last count there were 24 possible causes and fixes] for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.





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

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

u probably know all this but as far as i know u can't run stock injectors or stock maf with ur HCI. im guessing ur at around 300hp right. i know they say 19lbs r technically good up to 300 but that and a stock maf may be the culprit. also do u have a 255lph fuel pump?

Did you set the valves right when you did the H/C/I?

Stock shortblock?

I had a simular issue with the previous engine in my 93 doing the same thing. Turned out to be a cracked piston and a cylinder that would not seal the rings. Engine had a ton of blow by and would not hold steady RPM.

If you have replaced the intake manifold, the odds are that the gasket between the lower intake manifold and cylinder head slipped out of place when the lower intake manifold was put in place. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.

You guys with idle problems could same a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix you idle problems without spending a lot of time and money. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :

How bad do you want to fix your car?

If you have the symptoms of a vacuum leak, but can't find by squirting fluid on the outside, then the leak must be on the inside between the intake manifold and the lifter valley.

i have had major idle issues before as well. car did not drive well, i had bucking at light throttle basically.

i took it in both times for a dyno tune and it went away. i know you have your own tuning software, perhaps post up on a more technical tuning site. or try calling andy @ tms. he got my car to run pretty good from top to bottom with a 1 hour dyno tune.

it's so hard to diagnose these pieces of crap with idle issues. i had a surging idle before from a bad alternator. i had it before from a dirty iac, maf, alot of times it fixes itself then starts again. i know its frustrating as hell.. believe me, i've been there.

if worse comes to worse, i will drive down and help you look it over. your not too far away.

have you started from scratch at setting your base idle?

i know my car does not idle properly with the idle set below 900 rpm's. you mentioned a 700 rpm idle. have you tried raising base idle by setting idle screw, tps? we have the same camshaft, tfs stage 1.

that was throwing my car for a loop on the dyno. we slowly raised and raised the idle until it was returning properly. i had alot of the same issues until then. it would hot start different then cold start, it would start up give one surge, then just die, ect.

I am sorry to hear that replacing the intake manifold gaskets did not fix your low vacuum problem. Diagnosis is made using the information provided since I can't be there to observe things in person.

The man causes of low vacuum are intake leaks, misadjusted valves, and retarded ignition timing. You have replaced the intake manifold gaskets , so that reduces the possibility of leaking gaskets to just about zero. That leaves valve adjustment and ignition timing.


Putting the distributor back in and setting the timing.
Putting the distributor back in is fairly simple. Pull #1 sparkplug, put your finger in the sparkplug hole,
crank the engine until you feel compression. Then line up the TDC mark on the balancer with the pointer on the engine block.

The distributor starts out with the #1 plug wire lined up at about 12:00 with you facing it. Align the rotor to about 11:00, since it will turn clockwise as it slides into place.

Align the distributor rotor up with the #1 position marked on the cap, slide the distributor down into the block, (you may have to wiggle the rotor slightly to get the gear to engage) and then note where the rotor is pointing.
If it still lines up with #1 position on the cap, install the clamp and bolt. If not, pull it out and turn 1 tooth forwards or backwards and try again. Put the #1 spark plug back in and tighten it down, put the clamp on the distributor, but don't tighten it too much, as you will have to move the distributor to set the timing. Note that there is no such thing as one tooth off on a 5.0 Mustang. If it doesn't align perfectly with #1 position, you can turn the distributor until it does. The only problem is that if you are too far one way or the other, you can't turn the distributor enough to get the 10-14 degree optimum timing range.


Setting the timing:
Paint the mark on the harmonic balancer with paint -choose 10 degrees BTC or 14 degrees BTC or something else if you have NO2 or other power adder. I try to paint TDC red, 10 degrees BTC white and 14 degrees BTC blue.

10 degrees BTC is towards the drivers side marks.

Simplified diagram of what it looks like. Not all the marks are shown for ease of viewing.

ATC ' ' ' ' ' ' ' ' ' '!' ' ' ' ' ' ' ' ' ' BTC
---------------- > Direction of Rotation as viewed standing in front of the engine.

The ' is 2 degrees.
The ! is TDC
The ' is 10 degrees BTC
Set the timing 5 marks BTC. Or if you prefer, 5 marks towards the driver's side to get 10 degrees.

To get 14 degrees, set it 7 marks BTC. Or if you prefer, 7 marks towards the driver's side to get 14 degrees.

The paint marks you make are your friends if you do it correctly. They are much easier to see that the marks machined into the harmonic balancer hub.

At this point hook up all the wires, get out the timing light. Connect timing light up to battery & #1 spark plug. Then start the engine.

Remove the SPOUT connector (do a search if you want a picture of the SPOUT connector) It is the 2 pin rectangular plug on the distributor wiring harness. Only the EFI Mustang engines have a SPOUT. If yours is not EFI, check for a SPOUT: if you don’t find one, skip any instructions regarding the SPOUT
Warning: there are only two places the SPOUT should be when you time the engine. The first place is in your pocket while you are setting the timing and the second is back in the harness when you finish. The little bugger is too easy to lose and too hard to find a replacement.

Start engine, loosen distributor hold down with a 1/2" universal socket. Shine the timing light on the marks and turn the distributor until the mark lines up with the edge of the timing pointer. Tighten down the distributor hold down bolt, Replace the SPOUT connector and you are done.

The HO firing order is 1-3-7-2-6-5-4-8.
Non HO firing order is 1-5-4-2-6-3-7-8




The Trick Flow heads require different rockers: the stock pedestal rockers won't work, you need stud mount rockers). The stud mount rockers require different length push rods to get the valve geometry correct. Getting the push rod length correct requires some fiddling around an adjustable length test push rod and a test valve spring (reduced spring tension to work with the adjustable test push rod.) See http://static.trickflow.com/global/images/instructions/instruction sheet.pdf for the Trick Flow instructions

lancec2c30 said:

Update:

Just out of curiosity, I pulled a plugged line off my vacuum tree, and the car runs fine. What would be causes for a car to run better with a big vacuum leak?

I figured it was because of the extra air. So I opened the bridge between the ports on the IAC like suggested for cammed cars. But it still runs like garbage. With the IAC both plugged in, or unplugged.

Click to expand...

I'm in the same boat as you , i created a small vac leak from my intake and started up without a problem, i do get the dreded idle surge from time to time so i went ahead and got new 2 new ngk o2 sensors along with fords idle air adjuster kit # F2PZ-9F939-A. Worked way better on my h/c/i car and only surged once from having it on.