Help!! Pony Stutters when cold start and WOT

wmburns · Aug 12, 2010

BAP settings?

The TPS is not used the same way in a MAF car vs speed density. For MAF operation, the TPS (for non-limp in mode) mainly performs Closed-throttle, part-throttle, and wide open throttle determination.

In limp-in mode, the TPS can be used to replace missing sensor data.

IMO, the simplest way to determine if your TPS is working is to monitor the TP-MODE PID. It should read "P/T" (part-throttle) when your foot is on the gas. It should read "C/T" (closed throttle) when your foot is off of the gas. Now I do not remember what the value is for WOT, but it is easy to figure out.

So drive around and monitor the TP-MODE. If it matches what is actually going on, then it is most likely OK.

However, from your description, I think your TPS is working just fine. If it wasn't the car would likely have a "high idle" symptom. Further the fact that you can notice a difference between 98% and 100% also suggests that the PCM is getting a WOT signal from the TPS.

Consider that when WOT is activated, the PCM goes from "closed loop" operation to "open-loop" operation. At this point, the tune is a "speed density" tune running mostly off of tables.

If rich at WOT is now the primary symptom, consider that a spike in fuel pressure may be the underlining cause. What are the settings on your boost a pump (BAP)? Could this be causing the rich condition at WOT?

With regards to you MAF, it would help to datalog to see if it is maxing out. Note, the typical symptom for a MAXed out MAF is a LEAN condition.

SpazLE995 · Aug 12, 2010

I am running 20% and I thought of this while doing a road test so I put it to 10% but still doing the same thing. Then tried 0% and same thing.

Could there be a problem with the MAF?

I talked to Kenne Bell about the Air pump code and they said it will have no effect on the drive ability.

What would happen if I got a larger Throttle body?

SpazLE995 · Aug 12, 2010

Did some Research and came across this.

Midnight2V said:
Both cars use return style systems. The pump in the Cobra is the single 170lph unit, the GT has a single 110lph pump (the 98 got the cobra pump). Fuel lines are the same. The fuel rail on the GT actually flows more than the stock cobra rails and can support a little over 500rwhp. The GT sports 19# injectors while the cobra has 24# units, both are EV1 style. Both systems are set to operate at 39.15 psid.

Since I swapped the Cobra motor into the GT and did not realize the fuel pump was different flow capacity, would this have an affect on it? Wouldn't that be making it lean if anything rather than rich?

What would swapping a better fuel pump do to help the problem?

wmburns · Aug 12, 2010

If the motor is running rich and the fuel pressure is already above normal, how can swapping to a bigger fuel pump help?

If the pump is not keeping up, the symptom will be a lean condition and the fuel pressure will drop. The BAP is very effective about getting more out of any fuel pump.

OBTW, how do you know the motor is running rich? How has the tune been modified (or MAF changed) to prevent the MAF from maxing out? How are you monitoring your fuel pressure and has it been corrected for intake manifold pressure (delta)?

If you are measuring fuel pressure relative to Atmospheric and the motor is actually running lean, then the situation is different. Plus this would mean that the FP is actually low at WOT.

wmburns · Aug 12, 2010

SpazLE995 said:
Thanks for all the help guys, I am going to take a look at the fuel pump and see about deleting the PPRV.

Note, if you do the PPRV delete mod, this will remove the check valve in the fuel pump thereby guaranteeing that the FP will bleed down to zero between starts.

If FP bleed down might have been a problem before, it surely will be after. If done, expect hard starts after a cold soak or expect to have to key cycle before starting.

SpazLE995 · Aug 12, 2010

wmburns; said:
OBTW, how do you know the motor is running rich? How has the tune been modified (or MAF changed) to prevent the MAF from maxing out? How are you monitoring your fuel pressure and has it been corrected for intake manifold pressure (delta)?

If you are measuring fuel pressure relative to Atmospheric and the motor is actually running lean, then the situation is different. Plus this would mean that the FP is actually low at WOT.

I am running an Autometer Fuel Pressure gauge off the fuel rail.

I pulled the plugs out and they have carbon build up on them.
unless there is a problem with the ignition side wouldn't it be running rich?

would the car be running well other than WOT if there is an ignition problem?

I plugged my scanner in to see the Values for the Throttle body.
Fully Closed:16.9%
Fully Open: 92.9%
The value about where I back off the throttle from WOT to regain power is about: 78%

Is there a reason those values are not 0-100%?

wmburns · Aug 12, 2010

SpazLE995 said:
I am running an Autometer Fuel Pressure gauge off the fuel rail.

I pulled the plugs out and they have carbon build up on them.
unless there is a problem with the ignition side wouldn't it be running rich?

would the car be running well other than WOT if there is an ignition problem?

I plugged my scanner in to see the Values for the Throttle body.
Fully Closed:16.9%
Fully Open: 92.9%
The value about where I back off the throttle from WOT to regain power is about: 78%

Is there a reason those values are not 0-100%?

Houston, we have a problem.

If using an external fuel guage, the measurement will be atomspheric. The number needs to be adjusted by the amount of boost. How much boost are you running? Let's say its 10 PSI. If the external FP guage is reading 45 PSI, the the TRUE DELTA pressure is 35 PSI. This is low.

The manifold referenced (delta) fuel pressure needs to remain in the 38-42 PSI range.

Since you have access to a ODB2 data logger, the FP measurements need to be repeated using the PCM reported FP. I suspect the results will be that the FP is low at WOT. If so, this is the source of the problem.

Fuel pump upgrade anyone?

OBW, for the TPS, don't look at the load value. The numbers that you see are typical. Look at the TP-MODE.

OBTW2; It is not a good idea to base the assumption the motor is running rich at WOT by just looking at the plugs. There are other indicators of how the motor is running that should be used to cross check.

What if the motor is running rich at idle/part throttle? I presume that the engine spends much more of it's time running at part throttle rather than at WOT.

SpazLE995 · Aug 13, 2010

wmburns said:
Houston, we have a problem.

If using an external fuel guage, the measurement will be atomspheric. The number needs to be adjusted by the amount of boost. How much boost are you running? Let's say its 10 PSI. If the external FP guage is reading 45 PSI, the the TRUE DELTA pressure is 35 PSI. This is low.

The manifold referenced (delta) fuel pressure needs to remain in the 38-42 PSI range.

Since you have access to a ODB2 data logger, the FP measurements need to be repeated using the PCM reported FP. I suspect the results will be that the FP is low at WOT. If so, this is the source of the problem.

Fuel pump upgrade anyone?

OBW, for the TPS, don't look at the load value. The numbers that you see are typical. Look at the TP-MODE.

OBTW2; It is not a good idea to base the assumption the motor is running rich at WOT by just looking at the plugs. There are other indicators of how the motor is running that should be used to cross check.

What if the motor is running rich at idle/part throttle? I presume that the engine spends much more of it's time running at part throttle rather than at WOT.

How does boost affect Fuel pressure? I have a 6lb kit but for some reason it is only making about 4lb.

Flghtmstr1 · Aug 13, 2010

wmburns said:
Houston, we have a problem.

This man is wise. Heed his advice.

SpazLE995 said:
How does boost affect Fuel pressure? I have a 6lb kit but for some reason it is only making about 4lb.

Think about how a port-injected EFI system works. The fuel is injected into the intake manifold, as close as possible to the head's intake port. The air in your intake manifold is pressurized at 4psi. Since the fuel injector's nozzle is exposed to this pressurized air, the fuel has 4psi pushing it back into the fuel system when the injector opens. Therefore, the fuel rail pressure needs to be 4psi higher in order for the injector to inject the proper amount of fuel. Our cars have fuel pressure transducers that reference vacuum (and boost), so the PCM only "sees" the nozzle pressure. It is not aware of, and does not care about, the fuel rail pressure.

In other words, the fuel rail pressure is immaterial. The actual fuel pressure at the nozzle is the number that matters, and that number is what the PCM sees.

trinity_gt · Aug 13, 2010

SpazLE995 said:
How does boost affect Fuel pressure? I have a 6lb kit but for some reason it is only making about 4lb.

Fuel injectors are designed to operate with a pressure drop across them, usually about 40PSI. That means that the fuel pressure in the rail at the top of the injector has to be ~40PSI higher than the injector outlet sees in the manifold/port.

When you add a blower, you pressurize the intake to, say, 6psi. In order for the fuel injectors to continue to deliver their rated flow, the pressure on the rail side has to be 40+6 or 46PSI. If you go to 10psi, you'd need 50PSI rail pressure etc.

Return-style systems use a fuel pressure regulator in the line returning to the tank. By restricting flow back to the tank, it maintains pressure in the rails. But it has to be referenced to the intake manifold pressure to "know" what the rail pressure should be in relation to the manifold pressure. This is usually done with a "vacuum line" running from the manifold to the pressure regulator. You need to make sure this line is in place, is in good shape (no leaks, no kinks etc).

In a returnless-system (like the newer cars), the PCM senses the rail pressure referenced to the intake manifold via the pressure sensor and adjusts the fuel pump drive to maintain the reading at ~40PSI. When the fuel pressure drops, the PCM commands more drive to the fuel pump, increasing volume and thus maintaining fuel pressure.

SpazLE995 · Aug 13, 2010

Thanks for clearing that up makes sense.

I am going to change out my coil packs to see if that helps the issue.

Would removing the IMRC contribute to the loss of 1 to 2 lb of boost since it can flow more?

Help!! Pony Stutters when cold start and WOT

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