Resolved Stumble issue upon acceleration (1987 GT)

FoxDude87 · Sep 18, 2024

Before I try another volume test, I'll do some math with the results of my first volume test.

I got 75 grams of gasoline in 15 seconds.

Multiply by 4 = ~300 grams of gas per minute

300 grams of gas per minute x

Mcmahst said:
Maybe a dumb question, but have you verified the available voltage at the pump during the test? Not, what does it read with you test it static, but the running voltage? I found an insufficient ground problem testing a pump that way. Since these vehicles are so ground sensitive, I offer that as a test input that might point to a different problem. Just another thing to test before replacing the pump.

Hello, I have not verified the voltage at the pump during the test. It is possible that a ground could be off? Thanks for the idea.

Anyway I can test the running voltage without removing the fuel tank ?

AeroCoupe · Sep 18, 2024

You can back probe the wire in the connector behind the rear bumper, under the car, and just above the fuel tank.

This is the connector pulled apart:

You can see in the very upper center of this picture:

My guess is the fuel pump wire colors will be pink/black for positive and black for ground.

Mcmahst · Sep 18, 2024

Yes you can, but if you do a retest, I would run it more like one minute, not a few seconds. You’ll get a way more accurate result with a longer test. I’m sure others will chime in with better advise. Good luck.

FoxDude87 · Sep 19, 2024

Mcmahst said:
Yes you can, but if you do a retest, I would run it more like one minute, not a few seconds. You’ll get a way more accurate result with a longer test. I’m sure others will chime in with better advise. Good luck.

I will find a gallon container and run a 1 minute test and see what I get for fuel volume.

FoxDude87 · Sep 19, 2024

I will run an engine vacuum test soon. Where is the best spot to plug in the vacuum testor to the intake ? Any specific results I should be looking for regarding this test?

Mcmahst · Sep 19, 2024

There are several sites with good write ups on vacuum gauge testing. Here’s one that fits the bill:

How To Diagnose Common Engine Problems with a Vacuum Gauge

One of the easiest, cheapest, and fastest ways to check an engine for serious issues is to perform a vacuum gauge test...

m.roadkillcustoms.com

FoxDude87 · Sep 19, 2024

Mcmahst said:
There are several sites with good write ups on vacuum gauge testing. Here’s one that fits the bill:

How To Diagnose Common Engine Problems with a Vacuum Gauge

One of the easiest, cheapest, and fastest ways to check an engine for serious issues is to perform a vacuum gauge test...

m.roadkillcustoms.com

Thank you very much for this.

Tonys96Cobra · Sep 19, 2024

Did you replace the distributor trying to fix this problem? My 89 did the exact same thing yours is doing, my brand new distributor was the problem. Put the old Ford one back in and it fixed a multitude of issues I was having.

FoxDude87 · Sep 19, 2024

Tonys96Cobra said:
Did you replace the distributor trying to fix this problem? My 89 did the exact same thing yours is doing, my brand new distributor was the problem. Put the old Ford one back in and it fixed a multitude of issues I was having.

I did recently have a new distributor from LMR installed ( it cost me like $80)
I still have my old distributor. It's the original Ford Motorcraft.

I guess It's worth a try. What exactly was your 89' fox doing And what multitude of issues did you have?

I'll laugh if that's what's been causing my grief and confusion

AeroCoupe · Sep 19, 2024

Does the old stock distributor still have the TFI module on it? If so I would remove it and use some brake cleaner to remove the paste off the back of the TFI and the surface on the distributor. Then apply some Arctic Silver 5 AS5-3.5G Thermal Paste to the distributor surface and reinstall the TFI. This will ensure that the TFI has good thermal transfer to the distributor to keep it cool. I have seen where the thermal transfer was not good and the TFI module would be intermittent due to the heat buildup and then eventually fail. Link to the paste on Amazon:

Amazon.com

FoxDude87 · Sep 19, 2024

AeroCoupe said:
Does the old stock distributor still have the TFI module on it? If so I would remove it and use some brake cleaner to remove the paste off the back of the TFI and the surface on the distributor. Then apply some Arctic Silver 5 AS5-3.5G Thermal Paste to the distributor surface and reinstall the TFI. This will ensure that the TFI has good thermal transfer to the distributor to keep it cool. I have seen where the thermal transfer was not good and the TFI module would be intermittent due to the heat buildup and then eventually fail. Link to the paste on Amazon:

Amazon.com

Yes. The old original distributor still has the original motorcraft TFI module. I also have some thermal paste I purchased from Best Buy not too long ago. It's not the Artic Silver 5 brand you speak of but I think it should be good enough.

FoxDude87 · Sep 19, 2024

Tonys96Cobra said:
Did you replace the distributor trying to fix this problem? My 89 did the exact same thing yours is doing, my brand new distributor was the problem. Put the old Ford one back in and it fixed a multitude of issues I was having.

Also, where did you buy your distributor. The one that you later found was the problem?

Noobz347 · Sep 19, 2024

FoxDude87 said:
Hello everyone. I'm new to StangNet, so I hope I can find a solution to my driving issue.

I have a stock 1987 GT convertible Automatic 5.0 (speed density) car. I've owned it for more than 12 years with my fair share of ups and downs and I love this car.

It has 66-67,000 original miles on the clock and I'm having an acceleration issue. It seems to stumble (maybe misfire?) On moderate acceleration right around the 1500-1800 rpm range.
It does this when the engine is cold or warmed up. If I get on it, the rpms will drop up and down just slightly, Then she will smooth out.

Recently a New distributor, cap, rotor, motorcraft TFI module, plug wires, motorcraft copper spark plugs (gapped .054), Ignition coil, 3G Alternator, starter + starter solenoid, IAC, TPS (set to under 1 volt , about 0.72 volts. WOT is 4.25 volts)
EGR valve w/ EGR position sensor, PCV valve + grommet at rear of engine, ACT sensor, ECT sensor (coolant temp sensor), NTK oxygen sensors (no catalytic converters), fuel tank, fuel pump, fuel pump relay and filter.

Checked for vacuum leaks with a smoke machine and didn't find any leaks.

Idles good at around 750 rpm in drive. Idle is close to 900 rpm in park.
Timing is set to 10° TDC and it was set when the mechanic installed the new distributor, IAC and TPS.

It seems to have good fuel pressure when I checked it last. It does this almost every time I drive it now. If I'm steady on the gas, it's less likely to do it. And I've noticed it will stumble a bit more if I'm giving it more gas driving up a hill or inclined road.

Any ideas on where I should go next?
FPR ? Injectors? Transmission ? Lol

I looked through some of your other posts as well.

Thoughts: MAP sensor or a leak in the inlet someplace (previously mentioned). Also, look carefully for any vacuum lines that might be [sharing] a vacuum source with the MAP. If so, find a way a run the MAP sensor directly to the intake without any taps or Ts.

Also: Are the parts you swapped a response to this issue or did issues come after the parts swap?

FoxDude87 · Sep 20, 2024

Noobz347 said:
I looked through some of your other posts as well.

Thoughts: MAP sensor or a leak in the inlet someplace (previously mentioned). Also, look carefully for any vacuum lines that might be [sharing] a vacuum source with the MAP. If so, find a way a run the MAP sensor directly to the intake without any taps or Ts.

Also: Are the parts you swapped a response to this issue or did issues come after the parts swap?

I will look into the map sensor vacuum line or lines sharing that vacuum. I will see If I can run the vacuum directly to the intake by itself. Then take the car for a cruise.

This all started after a new distributor was installed. When I took car to the mechanic, he stated the distributor is good and is working as it should. He mentioned that I needed a IAC and TPS.

The car idles good , runs fine while driving to a degree but then seems to stumble intermittently.

RPMs will drop ever so slightly when I give it some gas while crusing around (or going up hills)

Almost "feels" like a misfire or fuel delivery issue.

If I get on the gas real good, doesn't really do it.

AeroCoupe · Sep 20, 2024

Reread all of this and wanted to see if you have checked the TPS to make sure the voltage is smooth and linear as it goes from idle to wide open throttle. This is a really good write up by Mike and in the first post you will see a referenced post by jrichker about TPS Setting and Trouble Shooting. Click on that and enjoy.

Thread 'Mustang TPS Adjustment - 0.99v not necessary'

Oct 17, 2010

EDIT: This post applies to 86-93 5.0 Mustangs.

Seems to be a common myth, so lets sticky a thread explaining how the TPS works. Jrichker has provided the basic TPS guideline to follow. But here's a snip of the A9L GUFB which is the programming logic. This is a Ford internal Doc (don't ask how I got it)

Pay attention to the part highlighted in yellow. I've posted a few more snips of the ECU GUFB related to TPS strategy in post #42 that highlight the ladder logic.

So with that said, adjusting your TPS to .99999999 volts is not necessary at all. The ECU has a...

FoxDude87 · Sep 20, 2024

AeroCoupe said:
Reread all of this and wanted to see if you have checked the TPS to make sure the voltage is smooth and linear as it goes from idle to wide open throttle. This is a really good write up by Mike and in the first post you will see a referenced post by jrichker about TPS Setting and Trouble Shooting. Click on that and enjoy.

Thread 'Mustang TPS Adjustment - 0.99v not necessary'

Oct 17, 2010

EDIT: This post applies to 86-93 5.0 Mustangs.

Seems to be a common myth, so lets sticky a thread explaining how the TPS works. Jrichker has provided the basic TPS guideline to follow. But here's a snip of the A9L GUFB which is the programming logic. This is a Ford internal Doc (don't ask how I got it)

Pay attention to the part highlighted in yellow. I've posted a few more snips of the ECU GUFB related to TPS strategy in post #42 that highlight the ladder logic.

So with that said, adjusting your TPS to .99999999 volts is not necessary at all. The ECU has a...

Mustang5L5

Replies: 47

Forum: Fox 5.0 Mustang Tech

Yes thank you for this. The TPS on my vehicle is 0.72 v at idle or (KOEO) and if I open the throttle , the volts slowly increases . There is 4.25 volts at WOT.

FoxDude87 · Sep 20, 2024

Okay everyone whos been helping me on this. I have done a couple tests.
I performed an engine vacuum test,
a fuel pressure regulator test
and another fuel volume test.

The engine vacuum test shows about 18 to 19" at idle. I tapped into the vacuum T mounted on the firewall. The needle is steady with hardly any real movement.
When I hit the gas pedal real good, the needle immediately jumped to 0 and then rebound to about 22" and then settled at 17 to 18". I have a video clip of this but I don't think it will post here.

I then checked the fuel pressure regulator without vacuum line attached. It shows 42 - 43 psi.

Finally , I did a timed fuel volume test by holding the vent button on the fuel pressure gauge. I got a total of 252 grams of gasoline in 1 minute.

So , I believe the engine vacuum is good
And my fuel pressure regulator is working as its supposed to.

As for the fuel volume, it stills seems less than what it should be. I'm thinking fuel pump may be worn out? I'm unsure.

Also, when looking at the engine vacuum needle, it's steady but slightly moving the Littlest bit between 17" and 18" . It's barely noticeable unless you focus on it.

Mcmahst · Sep 20, 2024

Vacuum looks great. 15 LPH sounds bad. Were you able to verify the running voltage at the pump? While it would be a low probability issue, I thought I’d ask.

FoxDude87 · Sep 20, 2024

Mcmahst said:
Vacuum looks great. 15 LPH sounds bad. Were you able to verify the running voltage at the pump? While it would be a low probability issue, I thought I’d ask.

I have not checked the voltage at the pump connector yet. I assume I just reach up near the fuel tank? Easy to get to? Maybe I can do this tomorrow with the KOEO.

Mcmahst · Sep 20, 2024

Yea, that’s right. As I said, it unlikely, but it’s free to test. Did you try the old distributor to see if that cleared things up? I’m sure others more knowledgeable will chime in on the pump issues.

Resolved Stumble issue upon acceleration (1987 GT)

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