Violent cutout at WOT. Not TPS or fuel...

[PutterMcGavin]

I recently picked up an '89 325i for $250 and have been working through a few items on the car. The newest issue however has me a bit stumped. Whenever going WOT the car violently cuts in and out essentially "bucking" the car before it eventually catches itself and accelerates normally. It doesn't matter if the car is off idle, at 2500 RPM or 5000 RPM at WOT it cuts out. If I was to go say 75% the car will carry all the way to redline in any gear so I suspect it isn't fuel related...

Thinking through things I've pretty much settled on the TPS being the problem. Reason being is that when I unplug the TPS the car accelerates without any of the hesitation all the way to redline (albeit without the ECU dumping in fuel).

Thing is though I tested the TPS and had the "click" off idle and also have continuity at both idle and WOT on the prongs. So suppossedly the TPS is working correctly. With the above in mind I'm thinking it could be one of the following:

1) ECU not giving correct signal for fuel
2) Fuel pump cant scavenge enough (although without TPS it will go to redline and seems to be RPM independent)
3) Some other problem that I'll keep trying to find...

...next thing I will do is test the fuel pressure under load to make sure it isn't cutting out. Any other ideas?

[jlevie]

If the WOT switch in the TPS is closing (electrical continuity) when the thottle is wide open, the TPS is working correctly. But make sure that the idle switch opens when the throttle is cracked.

I think my first suspicions in this case would be low fuel pressure/delivery (run the fuel system test suite in the Bentley manual) or a bad AFM.

[PutterMcGavin]

If the WOT switch in the TPS is closing (electrical continuity) when the thottle is wide open, the TPS is working correctly. But make sure that the idle switch opens when the throttle is cracked.

I think my first suspicions in this case would be low fuel pressure/delivery (run the fuel system test suite in the Bentley manual) or a bad AFM.

Thanks jlevie. The idle switch works as it should with the continuity going open upon cracking the throttle.

I don't have a Bentley manual yet (just bought the car not too long ago) but will order one online today. Any random tests you'd recommend until I get a hand on the procedures in the book? Thanks!

[Ray McCooney]

I would be suspect of the AFM as well. You might also want to check that the flap inside of the AFM isn't binding or sticking at WOT. I would clean it out and make sure that the flap is opening and closing in a smooth and proper fashion.

[jlevie]

The first fuel system test will be for pressure in the rail. That's done by teeing in gauge to the rail inlet. If idle pressure looks okay (~35psi) and it rises when you disconnect the vacuum line to the FPR to ~43psi, I'll clip the gauge to a windshield wiper and drive the car. At WOT the pressure should be ~43psi.

Low fuel pressure could be a clogged filter, line blockage, bad FPR, or bad pump.

[PutterMcGavin]

I would be suspect of the AFM as well. You might also want to check that the flap inside of the AFM isn't binding or sticking at WOT. I would clean it out and make sure that the flap is opening and closing in a smooth and proper fashion.

Good idea.

Just check the AFM and the flap moves freely without any hesitation, binding, clicking or anything. I cleaned it but have not run it so we'll see what happens.

The first fuel system test will be for pressure in the rail. That's done by teeing in gauge to the rail inlet. If idle pressure looks okay (~35psi) and it rises when you disconnect the vacuum line to the FPR to ~43psi, I'll clip the gauge to a windshield wiper and drive the car. At WOT the pressure should be ~43psi.

Low fuel pressure could be a clogged filter, line blockage, bad FPR, or bad pump.

Alright good thanks for the numbers. I was going to do this but have not gotten around to it yet. I have previously replaced the fuel filter with a new one so I do not think it would be that.

I will let you know what I find with the pressures once I get a chance to do that. Thanks again!

[twright]

Please be aware that I had a new fuel pump that would pass the Bentley numbers, but the car still barely ran above 3000 rpm. I drove the car with a fuel pressure gauge on it and found that the fuel pressure would drop to 10 psi above 3000 rpm. Another new pump and all is well (2 weeks of troubleshooting later.).

[PutterMcGavin]

Please be aware that I had a new fuel pump that would pass the Bentley numbers, but the car still barely ran above 3000 rpm. I drove the car with a fuel pressure gauge on it and found that the fuel pressure would drop to 10 psi above 3000 rpm. Another new pump and all is well (2 weeks of troubleshooting later.).

Thanks for the suggestions twright! I'm glad you figured out your problem and I'm sure it was a pain to have to replace a replaced pump!

I'm a little confused and thinking it is not the pump as the car WILL rev all the way to redline without the TPS plugged in. It may however not be dumping the volume of gas it normally would at WOT and thus the pump may not be working at full capacity. Who knows. I will def. check it out though!

[jlevie]

Thanks for the suggestions twright! I'm glad you figured out your problem and I'm sure it was a pain to have to replace a replaced pump!

I'm a little confused and thinking it is not the pump as the car WILL rev all the way to redline without the TPS plugged in. It may however not be dumping the volume of gas it normally would at WOT and thus the pump may not be working at full capacity. Who knows. I will def. check it out though!

With the TPS unplugged, the engine runs totally in closed loop mode. When the DME sees the WOT switch close it shifts to open loop mode using built in maps. Those maps will result in greater injector on time. So a weak fuel system can be a problem in that regieme and not when running in closed loop mode.

[E30Nova]

Not to be a dick, but cool! Someone else that has the same problem as me! ;)

I kid, I kid. It's just rare to see someone else have this problem. I'll be following this thread for sure.

Matt

[PutterMcGavin]

Not to be a dick, but cool! Someone else that has the same problem as me! ;)

I kid, I kid. It's just rare to see someone else have this problem. I'll be following this thread for sure.

Matt

Haha you're not being a dick at all. Glad I'm not alone in this!

I'm going to try to get some vids of the issue later today and will post them up. I'm thinking it MIGHT have to do with the fuel regulator so we'll see.

The good thing about having somebody else with this problem is that if I fix it doing one thing I can tell you and vice versa. Even if you or I try something and it doesn't work then the next guy know to try something else (assuming the problem is exactly the same...). So in a way we're each doing half the work right?! :p

[E30Nova]

So in a way we're each doing half the work right?! :p

Yeah, but I'm really lazy (working on cars all day kinda makes you not want to do it when you get home) so you might be doing my half of the work as well.

Matt

[Jean]

Checked the fuel pressure yet? Mine did the same thing, and it was fuel related.

[416stroker]

I had a similar plroblem when my pump took a shit as well

[delatlanta1281]

Try your ignition coil as well.