Fired up my son's GMC tonight after finally finding enough segments of time over the past few weeks to replace the headgaskets. The check engine light flickers if he lets the RPM's drop low while releasing the clutch. It didn't do that before-Any ideas? Otherwise runs great.

'87 2.8 S-15

 

is there a port you can plug a scanner into?

 

Does it have an oil pressure gauge on it? If not, you may want to put a mechanical gauge in to monitor oil pressure. Since it is flickering at low rpm, it shoulds like the oil pressure is too low. How low does the rpm dip when engaging the clutch off idle? Also, how high is the idle in neutral? It should be up around 700-800rpm at idle all warmed up. If it is not, you may want to increase the idle speed (if possible).

You should be able to grab a mechanical gauge and a 'T' to plumb it in where the stock sensor is. Again, if it doesn't have a gauge already, it probably only has a pressure sensative sensor that only triggers below a certain pressure. Back in '87, I think the CEL is really only an 'idiot' light, meaning that it won't tell you the problem, just that there is one somewhere.

Before I went to a mechanical gauge and a new sensor on my Chevelle, the idiot light would flicker at idle. Well, the motor was idling at between 350-500rpm, but it had 15-18psi of oil pressure at that speed. At anything over 800rpm it had 25+psi. That's what I get for putting in a high volume pump, not high pressure. I kept the idiot light, but got a sensor that would trigger at anything below 12psi.

 

Congrats on getting it fixed.

The cel should flicker if the rpms get too low. When the engine dies it should be on all the time. Id say you fixed another problem you didn't realize you had before. Providing it is getting plenty of oil.

My '87 has a oil pressure guage, if you want I'll check to see what the readings are for ya.

 

Oil pressure shows the same as it did previously 15-20 at idle and 35-40 at speed.

I'll have to check for a port on it. I have an OBDII scanner but not for the OBDI. The 94 Caravan I used to have allowed you to do the Key Dance after which the trouble codes would blink semi-morse code style. I haven't checked the manual yet but did GMC provide such a tool on these '87 vehicles or was it just more or less an idiot light as mentioned?

My son was babying it while we test drove it and let the RPM's drop pretty low by accident while releasing the clutch and then did it again just to show me. We'll keep an eye on it for a bit. The engine sounds great at this point and the idle is good. He left for school twenty minutes ago which is eight minutes away down the highway and no call from his cell so that's a good sign.

The thing that has surprised me about this engine is that it's broken four exhaust manifold studs so far. Three years ago the aluminum power steering bracket broke. It's a 1987 one model year only piece too. It attaches to the front two left side exhaust manifold studs. They were both broken off which put stress on the bracket and it cracked in two. That was a joy as one was broken off just below the surface. When we took the engine apart this time, the rear two on the right side were broken in the same manner. The top one broken off below the surface and the bottom one broken off where the nut was attached. It you lay the heads side by side, that would be the same position on both heads. Weird. That bracket seems to be common as the boneyard replacement broke six months ago in a different spot and when trying to find one, both times I noticed that all '87's in the yard were missing that bracket already. The first time I had to visit all five yards in town before I found one. This last time I was able to locate the last one available at the local Pick N Pull. Now I had one welded and am keeping it in reserve. The alignment of the pulley is good with the others and all mounting fasteners were in place the second time. I think I know why it was only used for one year.......



Helpful hint to everyone- a cheap way to mark your vacuum lines and electrical plugs is with photo lab "twin checks". They come on a roll like tape and consist of two matching number tags- one goes on your roll of film before processing and the other goes on the envelope with your name. They stick well as they're chemical resistant although I always wrap a layer of Scotch tape around the entire fitting or line just for insurance while lines are being relocated while work is in progress. I used to work in a lab so I have a roll that will last my lifetime but most labs will slip you a foot long strip of them free for the asking. They're easier to read than pen or pencil scratches on masking tape and there's no risk of accidentally using the same number twice if hand written. Cheaper than buying tags from the parts store.

Thanks everyone.