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1992 GMC Topkick - Weird No Start Problem, Help Neded

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Hi, thanks to all those who post her for the wealth of information provided.


I am almost the proud owner of a mint condition 92 TopKick C6000 with only 50k miles and  a 7.0L motor and Allison transmission. However the final part of the transaction is held up because of an issue we cannot fix. The truck had not been regularly used for about 9 years, but had been garaged by its 20 year plus owner, and would start easily and run, but cut out and then not restart until some later time, sometimes quite quickly, sometimes the next day. We have done some work in narrowing down the issue, but also in getting a few things in tip top condition anticipation that it's going to run and be used. The fuel filter was pretty plugged up so that was changed along with the in tank pump as that had a dirty and clogged filter and he flow was low. I checked inside the distributor and the ICM was a bit loose and one of the wires on to it was not too secure as the clip going over the tab was resting too high. Decided to change the ICM anyway and managed to get the plug latch to hold securely by using a cable tie to hold it down over the tab on the ICM. Having got it all together it fired right up and sounded fantastic, before the poor fuel flow had it sounding asthmatic. We were getting all excited and ready for a victory test drive when on closing the hood the engine cut out and would not start, it would crank, but no fuel was coming from the injectors, although the pump was running and fuel spurts out of the line at the filter if removed, much more vigorously than before the work on the pump and filter. This is what we battled with yesterday the truck would run, then die, then randomly work again when we took it around the local roads. We noticed that stopping often coincided with hitting a bumps in the road. Suddenly it decided it would go again after stopping today, so having noticed it had stopped once before on closing the hood we decided to test that theory to see if it was just a coincidence, sure enough a very gentle closing of the hood made the engine falter, with a mildly harder close the engine stopped dead. Same problem, engine cranks, has a spark, no fuel at the injectors, fuel pump running. We have checked all the wiring across the top of the firewall that is close to where the hood closes, it all looks like new the outer sheath is unmarked  and even pulling that back to inspect all the wires, we could find nothing. Looking at the maxi fuse panel there is nothing loose all the fuses etc. are as clean as the day the truck left the factory, however there are two relays mounted either side of the large fuses on the same bracket that holds the fuse box to the firewall. I have searched and searched to find what these do, to no avail, I'm wondering as the hood drops very close to the cover over them and the fuses if one of them is faulty and the slight impact is causing a problem. While we are close and now know why bumps in the road were killing the engine, presumably by jolting the hood, we couldn't get it to start again before we had to quit for the day, we felt as if we could get it running by wiggling some wire and tapping on the relays we would find whatever the issue was. Right now we are stumped other than something to do with one of the relays, but we know not what they control. Is there anything behind the dash that could be affected by hood closure? when we are closing the hood we are not slamming it, we are lowering it gently so the impact is very mild, not dropping it from a great height or anything.


Any suggestions greatly appreciated.

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The Truck fired right up this morning again, it seems it must be the Fuel pump relay from reading general internet threads on similar problems on various vehicles. However cannot find anywhere where it is located, I'm guessing its one of those on the firewall hence the hood closure creating the  "bump" to make it fail once it gets hot. Does anyone know where the fuel pump relay is located? I'm wondering if it is inside the truck on the firewall somewhere.

Edited by Redhawk4
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