Won't start w/o using gas pedal, I've done IAC, cleaned MAF, it gets fuel

[l88m22vette]

So, I'm having problems with my '98 3.1 SE. Over the winter I did LIM gaskets, fuel filter, IAC valve, and cleaned all the sensors. Recently, it had started to idle badly and would sometimes dies when stopping at a light. Now, I cannot start it normally, I have to use the gas pedal to get it to turn over and run and it will usually die once I let off the gas. Because of this I just cleaned the MAF again today, checked wires, and checked to see if fuel was coming out of the supply hoses, it is. From what I've read the most likely candidates would be either a clogged cat or a bad crank sensor, but would anyone have input on any other things to check or ideas? Thanks all

 

[Fivefingerdeathpunch]

If you havent done this yet, prime the fuel system and then take the vacuum line off the fuel pressure regulator. If fuel comes pouring out, then it's bad and can be the issue.

Also if you can remove the throttle body, do that and clean the throttle blade. If gummed up it can cause hard starts.

 

[l88m22vette]

Ok, I checked the FPR and no fuel came out, and the throttle body and MAF were cleaned a month or so ago when I did an oil change, so still no luck. When I did the IAC it sort of fixed the problem but obviously that doesn't matter with the no idling, I am going to check the crank sensor as long as I can with the weather tonight. Basically:

Tested/cleaned/replaced
- LIMs
- Fuel filter
- IAC
- MAF
- Throttle body
- Fuel lines (fuel is being pumped)
- Spark from all 6 plugs

To do
- Unscrew front o2 to test cat
- Crank sensor voltage
- Check for frayed/shorting wires under the car

Would a bad ICM allow the car to run when giving it gas but not allow it to idle? Any other obvious sensors or things to check? The entire problem is it won't start/idle on its own, but is fine when using the gas pedal. Cold start, warm start, it would idle roughly and stall when stopping, then it just stopped working.

 

[Steven Monteith]

Sounds like a similar issue I had with my 3.8. Check your fuses. I had a wire exposed to my headers and when it would touch it would blow a fuse, which was either the fuse for the MAP or MAF. Forget which one. Start checking fuses. I thought it was a fuel problem and ended up wasting a crap load of money thinking it was a fuel pump issue and ended up being a damn fuse.

 

[l88m22vette]

Ok, thanks, didn't even think about the fuses. I got home and was able to mess with it a bit before weather hit, I unplugged the MAF and the same behavior, so its not the MAF, however after I plugged it back in I was able to get the car started (again, using the gas pedal), and after keeping the RPM at about 800 I let off the gas slowly and its stayed running, though the RPM was a bit low and it definitely seemed like it wanted to die, and the exhaust smelled rich. For what its worth, while it was idling I unplugged the IAC and then plugged it back in and there was no change, as if the IAC wasn't part of the equation...

 

[Steven Monteith]

I did the same thing. unplugged those. Put in a new IAC and nothing changed. Definitely start pulling fuses. I forget what the fuses was labeled for. This happened over a year ago. The wires that were causing the short was the rear o2 censor. That links to all kinds of stuff. wiring diagrams get really consfusing. I had an electrical shop helping me out.

 

[l88m22vette]

Ok, so messed with more stuff, again the car seems to run fine with gas but dies at idle, but I noticed tonight that when I was using the gas pedal the headlights and interior lights would brighten or dim with revs, could it be a dying alternator?

 

[Steven Monteith]

Did you check your fuses? I could be wrong, but I think as long as you get the car started a crapped out alternator won't make it shut off...

 

[l88m22vette]

Sorry for the delay, I was out of town for a week. I've gone over the fuses and pulled any that could be related (ignition, fuel, etc.) and all of them look good. I was finally able to loosen the front o2 sensor but the manifold-to-exhaust pipe flange got in my wrench's way, I plan to pick up a sensor socket tomorrow to get the thing out so I can test to see if the cat is clogged. It wants to start right up but dies immediately, and I even disconnected to battery to see is that'd help, but no.

So tomorrow, aside from testing it with the front o2 removed (that's what you do for the cat test, correct?) I want to try to check the crank sensors. I also have access to an obd2 scanner, is there a chart for what all the readings (fuel trim, o2 volts, etc) should be? If I can see something is off that would help, and I found procedures for checking sensor voltages. Part of me just wants to take it in, but I know as soon as I find out what it was I'll be pissed lol

 

[l88m22vette]

Ok, so finally got back to trying things. The biggest thing I've tried is getting a new throttle position sensor, and still no start. Same issue, will run (badly) by starting and holding the gas pedal, but nothing as soon as I let off the pedal. Am I basically at crank sensor as the last resort? Anything else to consider?

Really, how would just starting/idling be affected if it runs with the gas pedal if I've replaced the idle control valve and throttle position sensor?