Got a 91 RS with a 3.1, giving me a HEADACHE. It will start and idle rough (almost dies) and won't go above 2k rpm when I give it a little gas. If I really step on it, the rpms will drop to nothing and it'll almost die again. The only code being thrown is 33, which is a MAP sensor. I've had it tested at 2 seperate shops, and it's coming up good. The vacuum hoses and lines look fine, and I'm running out of ideas...

Already changed plugs/wires, distributor cap + rotor, O2 sensor, fuel filter

Any ideas??

 

Have you checked fuel pressure? On that MPFI if one injector goes bad it will kill the whole side. Can you test for injector pulses? Have a noid light or a l.e.d. test light?

 

Haven't tested that yet, will try it today and see what I get. If it is an injector, how big of project will that be to fix? Just a little confused about it still giving me the MAP sensor code... could that have any effect on fuel pressure?

Thanks

 

does it have a mass air flow sensor? because that could be throwing the same things off if it is bad that happened to my dads truck and it wouldnt rev above 2200 and if you stomped it just died

 

did you actually hook a vacuum guage to the engine? Sounds more to me like you have a plugged cat. Manifold vac guage should read at around 15 when idling. crack the throttle and vac should drop. if you get it running and there is next to no vac or even worse there is pressure,( at idle)then look for plugged exhaust. If you simply unhook exhaust y pipe from manifold and it runs great ( other than the noise obviously) replace the cat. You can geat them installed cheap at most muffler shops.

 

I agree with shell29chevy, but also make sure the wiring to the MAP is good. Back probe the connector with a voltmeter. Connect (+) lead to the Gray wire and the (-) lead to the Black wire, you should see5 Volt with the ignition key on. If that is OK, connect the (+) lead to the Green wire and the (-) lead to the Black wire, with the engine runningshould see 1 to 2 Volt (high Vacuum)at idle, and 4 to 5 Volt at (low vacuum)WOT. If voltage remains low at WOT, the MAP is seeing high vacuum at WOT, whichis not normal. Since you said that the MAP sensorand hose are good, this leaves you with either a restricted exhaust, or a restricted intake.

 

Thanks for the replies, I've been really busy and out of town so I haven't had a chance to experiment with any of the suggestions. I took it to a local mechanic (because I don't see time opening up in the near future either), and this is what they say:

First, they thought it was a defective ECM. None of thier diagnostic equipment would work when connected to it, and it would explain why it's throwing an odd code at me. So, they checked all the wiring associated with the ECM, and decided that wasn't it after all (they got thier equipment working properly). Now, they tell me it's a fuel pump problem. I replaced that pump last year, so I'm somewhat skeptical about that...

So now to my real question: My fuel pressure is good (we both agree on that), but the mechanic is telling me that even though it has good pressure, it's not pumping enough volume? Now I'm not really up on my physics, but how can this be? Has anyone ever heard of a similar problem? All I know atm is I'm losing faith in this "recommended" mechanic really fast. Any help would be very appreciated.

 

Volume and pressure are two different things. When an engine is running under load it needs more fuel than at a lighter load. The fuel injectors are designed to work at a certain fuel pressure, and adjust the volume of fuel by changing the length of time that they are open. The pump may be able to provide adequate pressure under low load, such as idling, but may not be able to deliver the volumeof fuel needed underhigh loads, and this will show up as low fuel pressure under load. That being said, the mechanic should be able to verify the health of the pump, by driving the car around with a fuel pressure gage connected to the fuel line.

 

Well, looks like it is the pump (just changed it last year, it has less than 10k on it)... Gonna clear some time this weekend to get to it, any tips on making it as painless as possible?

Thanks for all the help

 

Yeah, Have some one else do it!
Jack up the car as high as you can and suport it underthe frame rails. Unbolt every thing from the rear except for the control arms and brake hose and let it pivot downwards. The brake hose is long enough to allow the rear to drop down far enough.Undo your exhaust (a good time now to install a flowmaster) heat shield,fuel lines, etcand snake the tank out