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90 C4 won't idle


90vetter

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Hello everyone newbie here and need some help with my Vette. It starts right up every time but stumbles and stalls. Service Engine Soon on, tried to run codes by jumping A and B at the diagnose port, at one attempt I did get a 46 (vats code) but it is crazy 12,12,12 12, 12 then it will flash a bunch real fast meanwhile lights flashing and very erratic. I did plugs,wires new battery,distributor module a couple months ago but now I can give it a little gas to keep it running and the service soon light on and dash stuff flashing and stumbling and it wants to die.Any help would be greatly appreciated. Thank You

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sounds like maybe your fuel pump mite be givin up .... would certainly check you fuel pressure just to confirm .... Oh yeah ..... and welcome to our humble abode !!!

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I'll have to check the fuel pressure but when accelerating all the way up to wide open throttle it has plenty of fuel and it doesn't miss a beat, only at idle or I gues at cruise it feels like a cylinder or two just drops out. Another thing while going down the road and when the miss feel occurs the speedometer goes from say 45 to 0 in one instant. I looked at the MAP sensor but not sure about it,I guess on the 90 vette they went from a maf to a map ?

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If they are original injectors? they were not designed to burn ethanol, fuel pressure also a possibility, do you have a factory service manual?, it will help

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Ground problem ????

Did C4's suffer from a crappy ground points like the C5 ?

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Try another module...it usually takes 2 before I get a good one in my Z/28 (TPI also)-even teh so-called "premium" brands are not teh same as they were a few years ago.

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Bromley's Corvettes

There is a main ground at back of driver side head. Its for the computer and other things I have seen guys leave that off and the car will run but have problems. There are lots of things that will make it not want to idle right. Vacuum leaks sensors computer problems. I would scan it and see what codes show up.

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:nothing:

JK...

When you check your fuel pump at the injector rails - if the needle is bouncing a lot, start with the fuel pump. Also, the connection for the fuel pump has been known to corrode and short - may need replacing.

Jim is spot on with the vacuum lines too. Fuel regulator is also known to blow out the diaphragm and crack the vacuum line which is hard to see.

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Wow lots of great stuff to try tomorrow . Yesterday when I tried to run codes the message center put out a lot of the codes that were bunched up and it must have put out so many 12's that I got tired of seeing them. Also while trying to retrieve codes the cooling fan came on then off then on etc, and the shift one to four light kept flashing. I disconnected the batt to erase codes but as soon as I started it up the service engine soon came on.It takes a while but if you stay on the gas enough to keep it running it will kinda idle and I went to the engine compartment to push and pull on stuff and when I moved the ECM it started to stumble.I kept playing with the connectors and wires to the ECM but couldn't say for sure it was there or that it was going to stall anyway

. First thing I do is look for that main ground then try everything else. Thank you all for ideas and your time to help me. I will tell you what happens. Thank you

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I checked the ground .all seemed good. they when I played with ecm , like twisted it a certain way, the engine did stumble.. and then twisted on each wire and tried to get it to stumble but don't know what that would condemn, the wires or the ecm. what inside of that ecm could go wrong? I do not have a pressure gauge to check the fuel pump pressure, but it sure seems like electrical.I tried to run codes again, but all I got was 12's forever and I know there is a hard code because the 'service engine soon ' light comes on and all I get is 12's... forever. When the last time my wife drove it,. she said that it started out chugging and then when it warmed up, the speedometer read 37,, then drop to zero, then 35.. all the while the car did not stall. checked the alternator too and that is good. it is also fairly new and the battery is brand new. we removed the positive cable when it was running and if alternator was bad then it would supposedly quit, but it kept running. hmmmm. maybe the ecm? ??any help would be appreciated .. thank you for putting the thinking caps on.. Neil

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Ok, I found out that I can get a ECM but along with it , it states that I may have to reprogram the PROM..is this something again that takes a special tool or can I do it from home at all? it states that the PROM must be transferred from the old unit to the new unit and relearn stuff, like theft, idle , crankshaft...etc. Neil.

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When the last time my wife drove it,.

There it is, the wife broke it!
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You will not need to reprogram the new ECM. You will remove the PROM from your old ECM and put it in the new one. Before settling on an ECM, be deliberate in your testing. By moving the wires, you have performed something called a wiggle test, this test is good for testing the integrity of the wiring harness and connectors. To check the integrity of the ECM, perform a tap test. Sharply and firmly use your fingers to tap the ECM while the car is running and see if you can get the car to glitch. If it does not, the problem likely does not lie with the ECM, but rather the wires you manipulated. It could also be as simple as a bad IAC motor or a worn TPS. Hope this helps.

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feel the wires one at a time around the ecm connector and pinch them, sometimes the wires can break inside the insulation which will give it a hollow feeling, also disconnect the ecm connectors and look for anything out of the ordinary, its a completely sealed connector(supposedly) more specifically look for any powdery residue white or green. even a little bit will cause fretting which is easily corrected with a little di-electric silicone grease. if there is a lot of fretting that borders on the line of corrosion you will need to find the water leak or replace the connector and pins which isn't fun but its not that hard either.

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Working at home with caveman tools I thought and thought what would cause all the things that I see with this baby and said I'm going to NAPA and get a reman ECM And I just installed it about an hour ago and it's running so sweet ! Thank you all for your input. This baby's back on the road :):):) Neil

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That's thinkin' with yer dipstick. Look forward to seeing the car at an event sometime :thumbs

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