Over the past few weeks I've been chasing a problem that is about to drive me to destruction. The car runs fine but under hard acceleration at about 4K RPM and at full boost is cutting out abruptly with a great deal of backfiring. If I back off the throttle it runs fine again. My sense is that this is electrical. So far I have done the following: replaced CV, coil, wires, plugs, cup and rotor. I've checked for vacuum leaks, electrical sorts, turbo pumbing and wastegate operation. All came back negative. There are a couple of things that we are still checking: crank sensor and TPS. If these check out, both my P-tech and I are out of ideas. The car is dead stock so it can run in PCA F-Stock. Ideas anyone? Thank you in advance.

 

Crank sensor and TPS checked fine. I'm out of ideas. Anyone? Anyone?

 

Have you checked the air flow meter to make sure it has a smooth linear signal when the flap opens?

 

My tech claims he checked air flow meter and it is fine. Tomorrow he will be checking fuel pressure but I seriously doubt this is the culprit.

 

Why do you say that? The symptoms sound very much like it could be a bad fuel pressure regulator.

 

Back Fire Awsome!!!! Only when its meant to be done.

 

Any chance you are hitting overboost protection?

Bad Fuel Pressure Regulator? Weak Fuel Pump?

Bad bypass valve?

 

There is a great deal of backfiring. That will tell me that there is unburned fuel in the exhaust. In my mind a bad fuel pressure regulator will cause the engine to starve for fuel at wide open throttle. I'm I wrong?

 

Mine was similar with a bad FPR and dirty fuel filter...rich fuel supply can do some strange things under boost. New FPR and filter problems all gone. I admit though I had a few vac leaks to track down first.

 

Yep, you're wrong! Fuel starvation is not necessarily the failure mode for a bad FPR. When mine went bad 3 years ago it was spiking up to about 6 bar pressure, causing the ECU to shut down fuel supply. Problem only showed up at WOT, in my case above about 5K rpm, with large amounts of black smoke belching out the tailpipe.

The good news, if that is your problem, is a new FPR is one of the cheaper repairs we ever have to make on our cars! Good luck, I hope you get is solved soon.

 

Maybe there is hope. I'll be checking for fuel pressure problems tomorrow. I hope it turns out to be the FPR. I will post an update tomorrow. Thank you for your suggestions.

 

My car was bucking wildly recently after I installed a turbo and bigger injectors. Not sure if this would be the cause of your problem but it may. Danno led me to this one so he gets the credit. You have a 16 to 17 year old car. I have removed the injectors and fuel rail about 5 times in the last few years. You will find threads on here indicating how the insulation around the two wires going into the fuel injector pigtail is all brittle and has crumbled away causing shorting of the injectors and irregular firing. You can test it with a volt meter and verify by actually slicing the rubber pigtail boot and looking at the wires.

 

As I understand it, when the FPR fails, it fails in "open" mode, meaning it floods the EFI with fuel and your exhaust gets sooty. YMMV.

At around $60, it's definitely worth a shot.

 

Ted just just had the same problem. Take out the cap and rotor and check them and test coil and test wires if you can.

But what you said is exactly the same thing ted wright was getting.

SO GO CHECK THEM

-Ian

 

An update in the saga of the “sick” 951 track car. To restate the problem, at WOP (4200 RPM) and at close to full boost the car is cutting out with LOTS of backfire. If I back off the throttle it runs fine again.

On Friday we did a little experiment. We disconnected one fuel injector at a time and we tried to duplicate the problem. We could not. The car ran fine, for a three cylinder car. As soon as all four injectors we plugged in, the problem was back again. I'm not sure what, if anything, this tell us.

My factory-trained, 20 year-experienced and very well regarded P-tech is completely stumped! He went through every possible test. He replaced coil, wires, cup, rotor, CV and the temp sensor. He checked fuel pressure, vacuum lines, turbo plumbing, MAF, bypass valve, FPR, crank sensor and just about everything and they all came back negative. The only thing that he can tell me at this point is that we may have a broken valve spring. I personally I don't believe (or I don't want to believe) that this is the problem. I dropped out of the Watkins Glen race two weeks ago and I did make yesterday’s Lime Rock race. The race season is going by fast but my car is stationary and my frustration is mounting.
Any thoughts? TIA