Weird and Worrisome Symptoms

[1987 GMC Jimmy]

Hey, everyone. So I'll get right to it. I've got some weird 305 symptoms. I've got lots of noises. Tapping, clanking, and huffing from the tailpipe in most driving or idling situations. It looks like I have a little oil blow by on the passenger side, which is really bothering me. I found oil pooled in the air cleaner, on the little mesh filter, on the air horn seal, and on top of the carb to ThermAC seal. The thing lost a quart or so in approximately a thousand miles. The PCV is good. No blue smoke at startup. No noticeable smoke while driving, but I did punch it when I was driving it around, and even though it responded well, it left a cloud of smoke behind me. It idles like garbage, rough and struggling, but it was a little too fast when I last ran it two weeks ago. I haven't touched anything, though. I checked for a chewed or unplugged plug wire and all was fine. I was experimenting and unplugged the electronic advance harness to see if it would mimic the base timing I set a few months ago, and the car wouldn't even start. When it idles it struggles to run and hunts for an idle speed. It smells rich as hell. The blow by that I saw wasn't huge that I could tell, and it chimneys smoke from the PS manifold area so I may have a leaky valve cover gasket, but that's not all of it. The huffing noise from the tailpipe is like when you cup your palms and then clap. This motor has approximately 63,000 miles on it, and it's a Jasper reman. It actually drove fine running errands today, but it idles too slow in park or at stop signs versus idling slightly too fast two weeks ago with no modifications to timing or curb idle. The noise and poor running is really vexing me, and I can't see any exhaust leaks to validate at least some of it. The fuel system is great from tank to carb, and the ignition system is less than a thousand miles old minus the distributor and ICM. Any ideas? I mean, could I have a blown ring on the passenger side at that mileage? If so, get it together Jasper. Jeez...

 

[Polyphemus287]

I would start with a compression test. If you have low compression in a cylinder, then throw a teaspoon of oil in cylinder with low compression and re run the test on that cylinder. If the compression jumps up you have bad rings. If the compression doesn't bump up then it may be a valve is not closing entirely. Definitely start with a compression test as there is point a throwing money at a worn out engine.

 

[1987 GMC Jimmy]

Well, I have a replacement for it on a build stand, but I wanted to get the most out of what I had in it already. I experimented with it today, and I found that there's some noise coming from the cat, which should solve some the clanking, but it still ticks, especially when it's cold. It has good power for what it is, and it never smoked. It did blow soot out the tailpipe, though. The PCV rattles, and that's what I was going by, but I'll replace it anyways and see what happens. I think I fixed the oil leak on it. I don't understand. One day it'll run like ****, and the next day it'll run great. It had 19-20 inches of steady vacuum at full warm up. Didn't diesel and ran smooth at about 650 RPM. It ran rougher and richer when it was cold, but that cleared up. I'll post the video of the tick when I get back to a solid wifi network.

 

[GTME94]

Did you mean that the cat is making weird sounds? Like clanking or metallic sounds?

Could it be plugged or partially plugged? A restricted exhaust will make an engine run poorly. Also a very restricted exhaust can force combustion gas through the rings and out the PCV system.

Something could be loose inside the cat causing the erratic behavior.

 

[1987 GMC Jimmy]

Did you mean that the cat is making weird sounds? Like clanking or metallic sounds?

Could it be plugged or partially plugged? A restricted exhaust will make an engine run poorly. Also a very restricted exhaust can force combustion gas through the rings and out the PCV system.

Something could be loose inside the cat causing the erratic behavior.

Yes, it is. I was able to finally isolate some of the noise to the cat, and it is the metallic clanking noise. Whenever I had the car in the paint shop, and they did a few extra things, gutting the converter was supposedly one of them. I don't know if they didn't do it right or get it all the way, but I can tap on it with a broomstick and hear junk rattle around. I do wonder if there's something funky happening with the exhaust causing it to run weird. I still have the steady passenger side tick, but it quiets down as it warms. Rockers and pushrods appear well on that side. Here's the tick and the clunk being projected through the tailpipe. I wonder how severe the affect of the cat is. My idle speed dropped about 250 RPM to around where it's supposed to be even though it's running rougher and noisier.

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[GTME94]

Can you disconnect the exhaust ahead of the cat and see how it runs for a quick test?

 

[1987 GMC Jimmy]

Can you disconnect the exhaust ahead of the cat and see how it runs for a quick test?

That'll be my next move when I get home.