85 c4 flashing engine light/stumble
Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
85 c4 flashing engine light/stumble
Hi folks, new to this forum and need some help. I bought an 85 c4 in October with 102,000 miles on it. It had been sitting for quite a while. Replaced O2, cold start, temp, and fan sensors, computer, ignition switch, cap, rotor, wires, plugs, fuel pump, air filter and oil change. Went thru the injectors and fuel rails. Also completely replaced the exhaust with a 2 1/2" custom setup with x pipe and flowmaster mufflers. Got it running well and after I replaced the exhaust and O2 sensor, we took off down the road with the check engine light on. Rode for about 20 minutes, didnt feel like it had much power. Stopped and had a beer then left about 30 minutes later. At this point it didnt want to run much at all and kept stumbling, at the same time the check engine light was flickering in time with the stumble. Then suddenly the check engine light went out and she ran like a scalded dog all the way home. When I started it today, it was fine for about 30 seconds and while sitting and idling, it will stumble and light will flicker. Any ideas on where to look and what to look for? Seems like electrical to me.
Safety Car
Joined: Jan 2004
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http://www.corvettephotographs.com/ecm.html
then disconnect the battery for a min and see if the code comes back
Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
I just went down and checked the codes - just 12 over and over again. So I started it back up and right away the check engine light was flickering and it was running rough. Also the tach was jumping all over the place. Thats a new one. I read on here somewhere that theres a ground near the oil filter. Maybe that has something to do with it? I know the ground at the back of the DS cylinder head is good. Which I forgot to mention that I replaced both heads with a set of 40,000mi World Products S/R Torquers with 2.02/1.60 valves. They came off of a solid running engine so I don't think they are the problem. But worth mentioning.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The ground that affects all that stuff is on the side of the block above the oil filter. The negative battery cable also attaches there. It's a well known source of electrical problems when it gets loose/dirty.
Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
Well I checked the ground by the oil filter, which was actually mounted on one of the bellhousing bolts. It was slightly loose so I tightened it up after taking a class on being a contortionist (hah). That didnt fix the problem. It runs fine for a minute then it sounds like the timing gets backed off severely so I unhooked the connector to check the timing, spot on at 6 degrees. Hooked connector back up and no change. So I unbolted the computer and tapped on the computer and messed around with the wires, no luck there. I unbolted the information center and messed around with those wires, no luck. The whole time I'm starting and stopping the engine so by now it was warmed up. At some point the engine cleared up and ran fine and I was unable to get it to run crappy like it was.
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Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
Thank you pmihaltian for the ground points write up. I checked the one by the oil filter and there is only the ground running from that point to the chassis. The only wires I can see coming from the harness are attached to a bellhousing bolt, which I toyed with for a while last night and didn't make a difference. I'm probably going to remove the exhaust manifold on that side to get a better picture of whats there. Today I started it and it ran fine again for about 30-40 seconds then started sputtering and rpms jump up and down. Almost like someone is cutting off power to the coil. I don't think there is water in the gas, I've had that happen to a different car and it doesn't seem like that is the problem. Weather permitting, I'm gonna continue looking for bad ground connections in the next few days
Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
One other thing - the interior lights don't come on when the door is open, which I've read is the dimmer module. I checked the ground to the module and it is good. Connecting the A and B pins brings the lights back on so I'm assuming the ground is good there.
Melting Slicks
Joined: Apr 2011
Posts: 3,496
Likes: 475
From: New Jersey
Check all the hots at the terminal block behind the battery. This is my soapbox issue since my C4 showed alternator symptoms/stalling issues and I found an incipient fire hazard at that block. Been fine since I replaced it and cleaned up some of the terminal ends that connected to it.
And check the pressure at the fuel rail, to eliminate that. You did replace the fuel filter, right?
One more question: any backfiring under acceleration when the issue is present?
And check the pressure at the fuel rail, to eliminate that. You did replace the fuel filter, right?
One more question: any backfiring under acceleration when the issue is present?
Racer
Joined: Oct 1999
Posts: 372
Likes: 1
From: Santa Fe TN
My '85 is doing the same thing, perhaps not as frequently as yours, but it sounds the same. It will run great, then usually when I'm on the interstate it will stumble, and the check engine light will flash twice very quickly, then all will be well again. Like yours, it seems like the ignition goes away for a second or two, then comes back. If I hook up the scan tool and check for codes before I shut down the engine, it will throw a code for "cal-pack missing. If I shut down the engine and restart later, the code goes away. It's currently awaiting a TV cable replacement (will probably do that this weekend) and I'll look again at the grounds and the block behind the battery. Mine is stock, 232,XXX miles, put in a Jegs crate motor (the basic Mr. Goodwrench replacement 350) about 40k ago. Sorry I don't have a solution, but if I figure it out this weekend, I'll report back...
Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
Late evening yesterday it was running fine so I took it around the block, no problems at all. I did notice after fiddling with some fuses that the "security" light stopped blinking with the doors open. Turns out the fuse was dirty. Today I went to the store for meds and it ran ok but idle felt weird. I'm gonna take all the fuses out, clean the blades and unhook the computer and spray the terminals on the connector with an electrical cleaner and see if that helps. I'll have to work on the engine grounds this weekend if the sickness lets up.
Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
@dinobob, it wasn't backfiring, more like surging under any pedal pressure. I have not replaced the fuel filter yet, that is next on the list although I don't believe I have a fuel problem. One more thing to eliminate though.
Last night I removed each fuse and cleaned the metal blades, wow was I surprised at how filthy they all were! I also disconnected the computer and sprayed electrical cleaner on the computer pins and the connectors themselves, there was a little green corrosion there, not much. Put it back together and it seems ok again. Will have to try to take it out later today to see if it made a difference. I do really appreciate everyone's input so far! I hope to get this resolved fast and start posting some of the custom mods I plan to do. The 2 1/2" exhaust looks pretty good from where I'm standing and it cost waaaay less than a full Corsa or Magnaflow setup.
Last night I removed each fuse and cleaned the metal blades, wow was I surprised at how filthy they all were! I also disconnected the computer and sprayed electrical cleaner on the computer pins and the connectors themselves, there was a little green corrosion there, not much. Put it back together and it seems ok again. Will have to try to take it out later today to see if it made a difference. I do really appreciate everyone's input so far! I hope to get this resolved fast and start posting some of the custom mods I plan to do. The 2 1/2" exhaust looks pretty good from where I'm standing and it cost waaaay less than a full Corsa or Magnaflow setup.
Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
After lots of messing around and fixing the dimmer module per Rob Healeys excellent write-up, I went back out to find that the interior lights quit working again. Left the door open for a bit and they flickered, then came on. Then they went out. So I think the ground that powers the interior lights is an issue, but I did get the check engine light to stop coming on and off. Guess I will take pmihaltian's advice and check every one of them. When this cold snap in Cincinnati breaks, that is.
Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
And now back to having the car run for a few minutes and check engine light comes on and car runs like ****. When this happens I use a jumper to connect pins A and B to check codes but it won't flash at all and the cooling fan does not come on. Still haven't gone thru each and every ground but I did check the one by the thermostat and the two up front, which were fine. UGH
Thread Starter
Racer
Joined: Jan 2013
Posts: 403
Likes: 87
From: Cincinnati OH
Yes that is exactly what i'm doing. So I connected pins A and B and check engine light doesn't flash and cooling fan doesn't kick on. I let it sit like that for about a half hour as I messed around with wires. Played with the wires for about 20 minutes, and nothing changed. After a while of not touching anything (half hour) the fan started kicking on and off intermittantly, and finally stayed on and the check engine light started flashing as it should. Started up and ran fine. Also the interior lights came on, which had been off the whole time. Any ideas on what to check here? I tried attaching a ground wire to the engine and touching various pieces of metal under the dash as well as some ground wires but that didnt do anything either. I'm about to rip out the whole damn wiring harness.