TheOtherEric

Fellas, my buddy drives a '91 C2T that started cutting out when he gets into the boost. I'm posting this as a favor to him since his car has been in two different shops continuously for 7 months now. I fear his mechanics just don't know how to diagnose such a problem.

The car idles fine, and drives fine, until he gets into boost. Then it starts stuttering. They've replaced a bunch of stuff to no avail, including plugs, coil(s?), fuel pump, turbo control unit, distributor, and rotor. Maybe even more. The car is stock, except for removal of a few sensors and things that his mechanic deemed unnecessary. Any ideas?

Kevin

Is he overboosting?

maverick

check the plug wires the boots are seperate to the wires and I had a similar problem that drove my mechanic nuts for a while. She was fine until you got on it and then it would stutter. Mine was only one wire....he even swapped out the cis system with another car etc etc.

TheOtherEric

He says they checked boost levels and they were ok.

He tells me his mechanic also checked the volume and pressure of the fuel pump, individually and as a system. Changed the gas, fuel filter, replaced throttle pos'n sensor, replaced all ducts, and checked boost levels. All to no avail, sadly.

nathanUK '81 930 G50

Air leak somewhere, leaking when on boost. Check all the hoses, gaskets, O rings etc.

Staffan

pressure switch okej ?

TheOtherEric

Since they replaced so many hoses, they don't think this is the cause. But that was my first guess too.
Thanks for the idea. I'll do some searching here to learn more. I saw in another thread that you stated headers might be an issue here. Well, he does have B&B headers, if that means anything.

nathanUK '81 930 G50

There is an overboost pressure switch. If the boost exceeds a certain amount it cuts off fuel to the engine. This can be earthed out, but make sure the car is not overboosting first. Use a plumbed in gauge first.

Staffan

Thanks Nathan, I should have been more clear on what I meant.
The pressure switch, located at the back of the intercooler left side just to the right of the boost sensor (orange thing), goes soft over time, so even if the pressure is okey the pressure switch might kill the fuel pumps. If you ground the cable connected to the pressure switch you will be able to tell if the switch is the problem. As Nathan points out you need to know for sure the car doesn't overboost before doing this.

DonE

The next step is to find out what AFR he is getting when you try to get boost. You've replaced everything else.

If the AFR is way rich, or way lean, then you've got a WUR problem - maybe as simple pinched vacuum line to the WUR or as bad as WUR replacement ($$$). Mine would do similar - stab the pedal and it popped/studdered and didn't go anywhere. My AFR's were lower than 9:1 - the WUR was dumping too much fuel.

TheOtherEric

Thanks for the ideas guys. We'll start devising a plan of attack. Any more thoughts are certainly welcome.

cobalt

This sounds like a similar problem DR Jup was having a while back. Check the seal from the IC to the throttle. If the IC was removed recently it may not have seated properly when installed.

Staffan

Another common similar problem is a leak between turbo and intercooler, seen it several times. Holes in the large rubber hose that is hard to see unless under pressure, broken hose clamp etc.

Also had leaks with similar symptoms at the CBV (hose back of intercooler left side).

Has a CIS test been done (fuel delivery, system pressure, control pressure, spray from injectors) ?

911 2

definitely a leak somewhere and i bet its going to be silly

TheOtherEric

They're pretty sure that it's not a leak issue. They've removed the IC several times and the problem persists. They also installed a boost guage to ensure it's not overboosting, and disconnected the overboost switch. Also replaced the WUR to no avail.

I guess they also tested the fuel system apparently extensively. They even logged fuel pressure, and apparently it maintains fuel pressure even while sputtering. So it sounds like a spark-related issue, and so they now want to replace the plug wires.