High RPM Stumble

mattmm04

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Apr 21, 2019
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I have a 98 GT with a engine from a 2000 GT. Swap went well and runs and drives great but have a slight issue when under WOT at high rpm ranger from 4-5K. Seems like combustion isn't good under load at high rpm as if it's stumbles ever so slightly.

So first the full story of the history of the car and just a small list of mods. Bought the car from a guy who drove it up until sometime in mid 2018 when he spun a bearing and created a bad knock on the bottom end. The car was parked for however long for the remainder ( the guy never said how long it had been sitting) until I bought it in January 2019. The car did have about half a tank of fuel sitting in the tank since he had parked it.

I replaced the engine with a 2000 GT with 157K used engine with a new PI intake manifold, new NGK plugs, new alternator, clutch, rear main seal, new stock flywheel, adjustable clutch cable and quadrant, water pump, a used SVE plenum and 75mm throttle body and a cold air intake with stock MAF sensor. Also have a Bama tuner running on the 93 octane tune right now. I will mention at this time that the stumble has always been present before changing the tune and after changing the tune the stumble may be slightly worse than before the change but the stumble is always present regardless whether it has the tune or not.

Now here's where I'm at presently.... I've ran about 3 or 4 tanks of 93 fuel through it from driving it in hopes to clear out any of the old gas that I'm thinking has been in for 6 or more months thinking maybe the old gas is giving me trouble. I feel like the old fuel should either be gone by now what is left has mixed enough with new fresh fuel would even it out some but still has a stumble. So this morning I finally got a chance to slap a fuel pressure test gauge on and went out for a small drive.

Idle I'm at 28-29 PSI... Casual driving 30-34 PSI.... WOT spikes to 39 but drops back to 37-38 PSI. I'm taking my readings off the port on the fuel rail on the driver side

I've looked around for information on what's to be considered normal for WOT and seen where some have said around 40 or a little higher but I'm not hitting 40 at all. I've been having issues with the fuel sending unit as gauge on the dash is always fluctuating even when I know it has a full tank. My first thoughts is replace the whole fuel pump assembly and then move forward from there if the stumble persists after that.

My question is what do you guys think? Do you think that the old fuel with the ethanol sitting in the tank for months took away some of the performance of the fuel pump causing not enough air/fuel mixture when at high engine speeds? Or is the old gas still in tank somehow and causing problems with combustion? I've also have this stumble to occur going up steep hills now which makes me think more the fuel pump isn't up to par and not providing enough fuel when under a heavy load.

My next steps before I pull the tank and change the pump is simple things such as clean the MAF sensor and change the plug wires as they are some cheap knock off brand I've never heard of and while I'm changing the wires I'll pull each plug while I'm at it to see what the plugs look like and maybe get a clue to what's happening during combustion. I also plan on changing the fuel filter. I still feel like the pump isn't pushing enough fuel but I would like some input to see if those numbers are ok and should start looking elsewhere before pulling the tank.. Thanks for any info!
 
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wmburns

SN Certified Technician
Aug 14, 2009
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397
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Houston Texas
Your fuel pressure is not all that bad. Only a PSI or two lower than it should be. The difference between 39 PSI and 40 PSI could be caused by differences in the measurement devices.

Just to make sure that you are aware that the fuel pressure is indexed by the intake vacuum reference line. The fuel pressure is supposed to be lower when the motor is idling. For a test disconnect and plug the fuel pressure regulator intake vacuum reference line. Does the fuel pressure jump to about 40 PSI?

It seems to me that there's a higher likely hood of your problem being:
  • tune related. You have a PCM that is tuned for a non-PI motor running a PI motor. There is a different timing curve between the two. The difference is more likely to show up under high load. I believe the non-PI motor likes more timing than the PI does. Plus the PI motor needs more fuel than the non-PI.
  • Fuel injector sizing. WHICH fuel injectors were used? The original ones or the ones from the donor motor?
  • Ignition related. As load increases the demands upon the ignition system go up. If there are any weakness in the ignition system this is when they will show up.
I would like to encourage you to take a logical approach to this problem. IF your problem is indeed fuel related this will also show up in the short term and long term fuel trim values (STFT LTFT). It will also show up in the O2 sensors. For example do the front O2's go lean when you get onto it?

Do you have an ODB2 scanner? If so, it might make short work of your trouble shooting and save you a ton from not changing good parts. Here's some information on an affordable ODB2 scanner with GRAPHING capabilities. The ability to graph multiple PID's may prove vital to confirming the problem.

ForScan ODB2 scanner w ELM327 USB

Note: ForScan is NOT a tuner. It can't help you to re-tune the motor or load custom tunes. It is a powerful diagnostic tool that will give real time information about what the PCM is seeing and how it's reacting.
 
Last edited:

mattmm04

New Member
Apr 21, 2019
15
0
1
33
Virginia
Your fuel pressure is not all that bad. Only a PSI or two lower than it should be. The difference between 39 PSI and 40 PSI could be caused by differences in the measurement devices.

Just to make sure that you are aware that the fuel pressure is indexed by the intake vacuum reference line. The fuel pressure is supposed to be lower when the motor is idling. For a test disconnect and plug the fuel pressure regulator intake vacuum reference line. Does the fuel pressure jump to about 40 PSI?

It seems to me that there's a higher likely hood of your problem being:
  • tune related. You have a PCM that is tuned for a non-PI motor running a PI motor. There is a different timing curve between the two. The difference is more likely to show up under high load. I believe the non-PI motor likes more timing than the PI does.
  • Fuel injector sizing. WHICH fuel injectors were used? The original ones or the ones from the donor motor?
  • Ignition related. As load increases the demands upon the ignition system go up. If there are any weakness in the ignition system this is when they will show up.
I would like to encourage you to take a logical approach to this problem. IF your problem is indeed fuel related this will also show up in the short term and long term fuel trim values (STFT LTFT). It will also show up in the O2 sensors. For example do the front O2's go lean when you get onto it?

Do you have an ODB2 scanner? If so, it might make short work of your trouble shooting and save you a ton from not changing good parts. Here's some information on an affordable ODB2 scanner with GRAPHING capabilities. The ability to graph multiple PID's may prove vital to confirming the problem.

ForScan ODB2 scanner w ELM327 USB

Note: ForScan is NOT a tuner. It can't help you to re-tune the motor or load custom tunes. It is a powerful diagnostic tool that will give real time information about what the PCM is seeing and how it's reacting.
I am using the stock injectors from the 98 and that’s why I posed the questioning that the fuel pump was good. Maybe I should talk to Bama and have them take the tuner and do a stock tune for what I have and see if that takes care of it? I’m glad you brought up the ecu for a non pi because that would definitely hinder timing and air/fuel mixture I would say. I just didn’t want to change the pump at this time as I want to get my gears installed. I do have a scanner but haven’t looked to see what happens to the o2s when under heavy load. I’ll check that today
 

mattmm04

New Member
Apr 21, 2019
15
0
1
33
Virginia
Your fuel pressure is not all that bad. Only a PSI or two lower than it should be. The difference between 39 PSI and 40 PSI could be caused by differences in the measurement devices.

Just to make sure that you are aware that the fuel pressure is indexed by the intake vacuum reference line. The fuel pressure is supposed to be lower when the motor is idling. For a test disconnect and plug the fuel pressure regulator intake vacuum reference line. Does the fuel pressure jump to about 40 PSI?

It seems to me that there's a higher likely hood of your problem being:
  • tune related. You have a PCM that is tuned for a non-PI motor running a PI motor. There is a different timing curve between the two. The difference is more likely to show up under high load. I believe the non-PI motor likes more timing than the PI does.
  • Fuel injector sizing. WHICH fuel injectors were used? The original ones or the ones from the donor motor?
  • Ignition related. As load increases the demands upon the ignition system go up. If there are any weakness in the ignition system this is when they will show up.
I would like to encourage you to take a logical approach to this problem. IF your problem is indeed fuel related this will also show up in the short term and long term fuel trim values (STFT LTFT). It will also show up in the O2 sensors. For example do the front O2's go lean when you get onto it?

Do you have an ODB2 scanner? If so, it might make short work of your trouble shooting and save you a ton from not changing good parts. Here's some information on an affordable ODB2 scanner with GRAPHING capabilities. The ability to graph multiple PID's may prove vital to confirming the problem.

ForScan ODB2 scanner w ELM327 USB

Note: ForScan is NOT a tuner. It can't help you to re-tune the motor or load custom tunes. It is a powerful diagnostic tool that will give real time information about what the PCM is seeing and how it's reacting.
I am using the stock injectors from the 98 and that’s why I posed the questioning that the fuel pump was good. Maybe I should talk to Bama and have them take the tuner and do a stock tune for what I have and see if that takes care of it? I’m glad you brought up the ecu for a non pi because that would definitely hinder timing and air/fuel mixture I would say
 

mattmm04

New Member
Apr 21, 2019
15
0
1
33
Virginia
Your fuel pressure is not all that bad. Only a PSI or two lower than it should be. The difference between 39 PSI and 40 PSI could be caused by differences in the measurement devices.

Just to make sure that you are aware that the fuel pressure is indexed by the intake vacuum reference line. The fuel pressure is supposed to be lower when the motor is idling. For a test disconnect and plug the fuel pressure regulator intake vacuum reference line. Does the fuel pressure jump to about 40 PSI?

It seems to me that there's a higher likely hood of your problem being:
  • tune related. You have a PCM that is tuned for a non-PI motor running a PI motor. There is a different timing curve between the two. The difference is more likely to show up under high load. I believe the non-PI motor likes more timing than the PI does. Plus the PI motor needs more fuel than the non-PI.
  • Fuel injector sizing. WHICH fuel injectors were used? The original ones or the ones from the donor motor?
  • Ignition related. As load increases the demands upon the ignition system go up. If there are any weakness in the ignition system this is when they will show up.
I would like to encourage you to take a logical approach to this problem. IF your problem is indeed fuel related this will also show up in the short term and long term fuel trim values (STFT LTFT). It will also show up in the O2 sensors. For example do the front O2's go lean when you get onto it?

Do you have an ODB2 scanner? If so, it might make short work of your trouble shooting and save you a ton from not changing good parts. Here's some information on an affordable ODB2 scanner with GRAPHING capabilities. The ability to graph multiple PID's may prove vital to confirming the problem.

ForScan ODB2 scanner w ELM327 USB

Note: ForScan is NOT a tuner. It can't help you to re-tune the motor or load custom tunes. It is a powerful diagnostic tool that will give real time information about what the PCM is seeing and how it's reacting.
My scanner that I had was just a basic so I went and got another one that can do the fuel trim and 02 check. What numbers should I expect to see from the fuel trims and etc.? The stumble is starting to become more persistent on the low end of the range around 1500-2000 rpm also. I did a check on any misfiring and everything was all good
 

mattmm04

New Member
Apr 21, 2019
15
0
1
33
Virginia
I just went for a drive and this stumble only happens when the engine is hot and going over the spark plug wires I forgotten that during the engine swap process I broke one of the coolant temp sensors next to the water neck of the intake and I notice of the live data that there was no input giving the ecu reading of -40.

My question now isn't looking at the fuel delivery but at the coolant system sensor and the advance timing read out. The read out I was getting on the scanner was at 40-45 degrees advance at any given load. Isn't that too much or is that normal?
 

wmburns

SN Certified Technician
Aug 14, 2009
5,209
397
164
Houston Texas
I'm not sure how to respond to your question. To me it seems completely plausible to me that the PCM will not be using an optimal fuel enrichment when the PCM "thinks" that the motor is soooooooooooo cold (-40 degrees F). One of the goals of the fuel enrichment strategy is to speed up the motor warm up speed.

If this were my car I wouldn't spend one additional minute worrying about what else "might" be going on until after the ECT sensor was repaired. WHY? Because of the amount of research that would be required to determine exactly how the PCM will respond to a missing ECT sensor. I could also suspect that there could be differences between the various model years.

But if you really want to know what's going on then look at:
  • O2 sensors
  • Long term and short term fuel trims (LTFT and STFT).
Perhaps graph during a run:
  • RPM
  • computed motor load
  • STFT
  • MAF
But if asking for an educated guess, look for evidence of the motor going lean during the stumble. Why? Because the PI motor moves about 40 HP more air than a non-PI motor. Where as the PCM is calibrated to deliver fuel based upon the non-PI motor's fuel curve.

It seems to me that the differences in the tune between a non-PI and PI motors is the most likely to show itself during periods of high load.

It also seems to me that this is also a "win" case study for what can be done with an ODB2 scanner.
 

mattmm04

New Member
Apr 21, 2019
15
0
1
33
Virginia
Problem has been solved. I’ve been pretty busy so the car had been sitting for a week or so and while it was sitting I had put in the engine coolant sensor but didn’t have time to take it out. Yesterday I finally got to take it out and drove less than a mile and turned around and came back as it wasn’t running as it should. While stilling running I opened the hood to see if anything was out of the ordinary. Every time I touched the intake tubing and that particular area I got zapped. I had also picked up some new Beldin plug wires when I got the sensor. Swapped wires and noticed a difference immediately upon start up. Engine started faster, engine response is faster, no stumbles at any engine speed under load. I had planned on changing the wires initially during the engine swap but winter time is slow times for me and extra money wasn’t available. Now that the car runs smooth I can move on the the next item of having the 3.73 gear set installed and finish the body pieces that need to be replaced so I can get the car painted back to the original black and I’ll be happy
 

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