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My 1990 gmc c1500 idles strange, it misses off an on, and the rpm gauge jumps erratically whenever it misses, but once I get the rpm's above 3000 it runs like a charm. I've been trying to fix this for some time. i've set the timing at 0 degrees with the est disconnected, Ive replaced cap, rotor, wires , two sets of plugs, cleaned the TBI checked all the sensors, changed all the filters, there are no trouble codes. I can't figure it out. also, I have an aftermarket fiberglass hood on the truck and I can't find the info that would be on the underhood label, with all the specs. Does anyone know this info for my truck or where I can find it? thank you

Optional Information:
1990 GMC c1500 5.7 tbi

Customer (name blocked for privacy)

A couple of things to check for are vacumm leaks and the tbi gasket famous for deteriorating also check the egr valve for vaccum leaks and to make sure it is closing all the way.If this has been helpful an accept would be appreciated or if you need more info let me know. Thank you Bill

Reply to Bill's Post: I've checked and tried all those things, and still nothing. Here's something I noticed. Whenever I set the timing with the EST disconnected it idles fine and and doesn't miss a beat and hits at 0 degrees everytime, but as soon as I hook up the EST, it's starts acting up, and the timing goes way above 12 degrees, it's almost at 12 o'clock and jumps all over the place. it still runs, but something is not right and I want to fix it badly. I hope this helps. thank you.

check the distributor for play in the shaft if okay check for play in timing chain.let me know

Reply to Bill's Post: I forgot to mention that I replaced a worn out timing chain, just recently and the distibutor is a fairly new msd billet distributor that looks fine. i replaced the timing chain with a cloyes double roller.

did you chang cam shaft if so what did you put in. If not does irradic idle coincide with chain replacement, if so may not be set up right.

Reply to Bill's Post: I did not change the cam, and I put the timing chain on correctly, and yes the idle did coincide with the chain replacement, but I did many things in a short period of time such as: the timing chain, new lifters, plugs, wires, cap, and rotor, filters. Basically a good tune up with a few extras. Anyways I wouldn't think that it would be the timing chain, if it runs perfect with the EST disconnected. I'm thinking more along the lines of ignition, electrical, or a sensor giving the ECM some bad feedback, but then again I'm not the expert. I have a Chiltons manual for my truck that I have been using to test all the sensors, and everything checks out, but I've heard these manuals are junk. I'm at a loss... Thank you

I'm here to help, not hinder. Just trying to gather all that was done prior to.I suggested timing chain because if alot of miles and if slack it would cause erratic timing at low rpms. I did not know it was new when I suggested it.I will get back to shortley with specs.

Reply to Bill's Post: No! I didn't mean it that way. I should have explained in a little more detail on prior work done, and I truly appreciate any advice you can give because obviously you done this longer than I have.

if problem was not there prior to new parts, I know this is going to be a pain in the butt, but try old distributer if available. In the meantime I will look up new distributor.

Reply to Bill's Post: the distributor was in the truck when I bought it so I don't have the old one, I didn't replace the distributor recently, but it still looks fairly new and everything looks fine with. I have the MSD 6AL ignition system which was also on the truck when i bought it, also I replaced the coil recently too. just to let you know I've had this truck for approximately 8 months, with a paper trail of reciepts and instruction booklets for everything that has been installed on this truck.thank you

Sorry to take so long to get back to you. If this misfires then rpm change. have you checked to see if certain cylinder(s) are misfiring? If everything ran fine before you may have installed a bad component from the box. The ecm controls timing once you reconnect after timing and will try to compensate for any variation.What brand of plugs are you using?

Reply to Bill's Post: I have tried autolite plugs because that was what i pulled out of it. Then I went to get another set, just in case I had a bad one, and the guy at the parts store recommended ac delco plugs because they match the factory plugs that come in gm vehicles, so I thought I would give a shot, but it still does the exact same thing. both times I had the plugs gapped at .045. because that's what my manual says for my year and engine model, but the guy at the store said it it should be .035, but I haven't tried it because I wanted to go with the manual said. And like I said before I don't have the underhood label to go off of, so I'm not sure what they should be. Thank you

Can you check for loss of spark or weak spark. And just for my refreshment of memory, this was not occurring before tune-up?

Reply to Bill's Post: correct, it wasn't happening before the tune up, and I've checked the spark and everyhting looks good.

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