not starting

[Layne02]

Hey guys, 87 GMC Sierra 305 tbi . Rebuilt engine, Ive had it running for a while , but when I put it in gear it would die or barely move. I had yet to set timing so decided to try to set it. Found the brown /black stripe wire and unplugged it. That's when the trouble started . I haven't gotten it to run since. plugged wire back up and still nothing. I am stumped, all I did was unplug that wire . What could that have done ? Any ideas?

 

[1987 GMC Jimmy]

Hey guys, 87 GMC Sierra 305 tbi . Rebuilt engine, Ive had it running for a while , but when I put it in gear it would die or barely move. I had yet to set timing so decided to try to set it. Found the brown /black stripe wire and unplugged it. That's when the trouble started . I haven't gotten it to run since. plugged wire back up and still nothing. I am stumped, all I did was unplug that wire . What could that have done ? Any ideas?

Unplugging the EST wire shouldn't do anything unless your distributor was 180* degrees off. No, I think there's an ignition or electrical problem. Does your ECM do the little self-test light when you first turn the key to run? Can you pull codes like normal? Is that a new distributor or was it an old one transferred from a previous motor? Sounds like it may be the ignition control module. I'd take it out and have it tested at an auto parts store. If that doesn't go anywhere, I may consider something with the distributor, coil, or the ECM. ECM problems can be strange, at least in my experience. I don't want you to spend a lot of money so I'd start by checking the ICM. Oh, check your grounds, especially the one on the rear of the driver side cylinder head to firewall.

 

[Layne02]

All the sensors were replaced , but I have been getting a code 34, which has to do with map sensor, I put old one back on and still get same thing. Like I said , it will start and idle all day in park( at least before this) , but it doesnt want to run underr load.

I mentioned the wire because that was only thing I did before the no start problem.

It has started up a couple times since this but just died out.

The injectors are spraying into tb so its getting gas.

What else could a code 34 be besides just the map sensor?

 

[Layne02]

I forgot to add, this truck ran ok before rebuild, just had buncha leaks and 30 years of gunk on it. So my thought is ecm and distributor are probably ok

 

[1987 GMC Jimmy]

I misunderstood. Cancel the ICM. I thought you said it wouldn't start since you did that with the wire. Did you ever run long enough get your base timing? Well, you replaced the MAP with no results. How about the vacuum line? Mine melted one time for touching the EGR in a not good place, and that was about the worst it ever ran. There could be a wiring issue there so try to trace as much as you can. When Code 34 happens, the computer is supposed to substitute a fixed pressure value and divert the MAP's job to the TPS. You could check that, but I don't think the TPS is the problem. Something really makes me suspect the ECM here, especially if you're still getting the same code with two different sensors. If you got a third one and it did the same thing, you verified as much wiring as possible, and your vacuum was okay, I'd be pretty certain on the ECM. The 7747 ECM is fairly simple. You can take it out, look at it, and see if it's burned anywhere if you want.

 

[1987 GMC Jimmy]

This is my old 7169 ECM that I saved from my '86 Caprice. It was running terrible, and I'd troubleshooted through everything but the computer. I pulled it, opened it up, and look what I found on the bottom left hand side.

 

[Layne02]

No , I was going to set the base timing when I unplugged wire, then it wouldnt start after that so I could do it, then I plugged wire back up and it still didnt want to start. So I never got to do it.

Would something like a missing ground cause the code 34 to pop up?

I have built several carb sbc and havent had any problems, this is my first build with a "computer " and sensors to deal with. I miss a carburetor.

 

[1987 GMC Jimmy]

No , I was going to set the base timing when I unplugged wire, then it wouldnt start after that so I could do it, then I plugged wire back up and it still didnt want to start. So I never got to do it.

Would something like a missing ground cause the code 34 to pop up?

I have built several carb sbc and havent had any problems, this is my first build with a "computer " and sensors to deal with. I miss a carburetor.

Insufficient or broken ground can cause a potpourri of symptoms. It's always a possibility, and I forget to mention it a lot, but it should be one of the first things you check. There are grounds on the intake, one on the back of the cylinder head, battery, ECM ground... They're everywhere. They're a really great thing once you get the bugs worked out. It can just be a pain in the ass to find and eliminate those bugs, but once you do, it's smooth sailing.

 

[Layne02]

Ok thanks for all your help, I will dig around in it tomorrow if its not raining here. Thanks again

 

[1987 GMC Jimmy]

No problem. Hope you get it taken care of.