Hi all, Dave here, problem is with a 90' Olds Cutlass Supreme 3.1L Vin T. It came to me dead. Found it to have bad wires, which consequently caused failure of the ICM. Replaced ICM, wires , plugs, 1 of the coils (cracked) and now it idles smoothly untill you touch the throttle, it then has a really bad hesitation and will die unless you feather the throttle. As the rpm increases it smooths out and will take off about mid throttle. Response is fair to good. The fuel pressure at the valve is 48-50 at idle, increases 8-10 psi when I stab the gas and it holds pressure when the key is off. Volume tested good.
I did read where a low coolant or bad coolant level sensor can give inaccurate voltage reading through the MAP. Is this possible?
I cant find any vac. leaks, I have since tried a different coil pack and ICM with no change also tried a TPS and IAC also pulled the upper intake to check injectors they all have nearly identical resistance. Checked the fuel reg for ruptured diaphram. Tried another ECM from a identical car, no change. Just removed the cat so its not a exhaust restriction. It has had new injectors less than 12 months ago. I am scratching my head on this one . I must be missing something any thoughts would help. Dave J.

Throttle body clean?

I did the carb clean hose down. I will go out and remove the IAC housing and check / clean thoroughly. I just R&R the throttle body so it has a new gasket. When I had it off it looked pretty clean in the throat.
The more I mess with this thing the more convinced I become that this is a fuel or major air leak issue. I dont believe that it is a injector problem, it idles smooth and without a load it runs so good except right off idle. It has plenty of pressure and it holds it, plenty of volume too. If its a air leak I cant find it. It doesnt smoke like a crankcase vac leak usually will. I am at a loss with this one.
Thanks for the input I will post the results of the housing clean.
Dave

have you fixed the gauge so you can see it driving?

Have seen the vacuum lines at the back of the intake gets switched around. This will cause drive ability issues, real easy to do on these engines.

jw firebird- not as of yet, I will though. I did watch it when the hesitation was occurring. It didn't change in pressure
chevy82- Hadn't thought of that and thanks I will check that out.

Update: Well the stumble was still very apparent so I got a wild hair and disconnected the fuel reg and the MAP vac. plugged them off and the hesitation is gone. Hmmmm. I suppose I will change them both and hope this problem goes away. Something tells me it wont.
Chevy82-
2nd. update: Its not the vac. lines they are right. Damn

Just went to confirm that the stumble was gone and nope its back with the map and the fuel reg. vac connected or disconnected. Not sure what was happening before. I did notice that immediately after starting it stumbles a little less than after it has run for a minute or so.

I checked the timing chain for slop. It feels less worn than the double roller on my hot rod. In other words, its fine. I should also mention that this thing has been to the GM dealer they couldnt figure it out either.

Just curious....how did you check the timing chain for slop? Only way I can think of is to pull the front timing cover?

are the vacuum lines old? could be dry and only sealing intermittently when you were messing with them. if they look like a tire that has been out in a the sun a while they should just all be replaced. they are cheap and not worth the hassle of just replacing one at a time.

jw: Can I call ya jw? Anyway, the vac lines all 3 are mostly hard lines, rigid plastic I mean. I wouldn't really call them a hard line but you know what I'm referring to. I did think one of them was bad the second time I pulled the upper intake off so I went ahead and replaced them all. I dont think they are an issue now, I have sprayed enough carb cleaner around them that a leak would surely show up with a change in idle.

Tech II: Are you kidding me? Im an old fart I suppose 30-35 years turning wrenches, I will just say there is more than one way to skin a cat. However since you asked, by feel grasshopper, by feel.

Well, that is kind of hit and miss, by feel.....

When there was a distributor, you could pull the cap, and check slop by rotating the crank back until the distributor started to move, then rotate it clockwise and see how many degrees on the timing marker it moved before the distributor started to move again......that'* a visual....by feel, kind of really guessing.....