Got me stumped

I've posted here before about a persistant miss on my '88 Z-24. The miss is on #3 cylinder (pulling plugwire off #3 doesn't change idle, plugwire arcs).

Miss is noticeable at idle more than at increased speeds but I sense it has less power than before during all rpms.

I have a donor car I robbed parts from to try on this:

1.) There were no codes.

2.) I replaced the corresponding coil pack although the plug was firing, still is. No change.

3.) I replaced ALL the injectors, although the old ones tested ok for resistance, & replacements all tested ok also.) No change, still missing on #3.

4.)Found the egr valve to be stuck, replaced (there were no codes for this). No change.

5.) Replaced the IAC motor. No change.

6.) Took compression check on that cylinder: 150 psi.

7.) Switched spark plugs between #3 and adjacent cyl; plugs look similar--whitish grey. No change.

8.) Heads were replaced new last summer.

9.) Finally took the wire loom that plugs onto injectors and swapped it with the one from donor car--then I could have SWORE that fixed the problem--and maybe it was my imagination--but after driving about twenty minutes, back to missing on #3 again.

ANY HELP APPRECIATED. Thx in advance.

Reply to
James Goforth
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Have you done a compression check on #3 yet?

Reply to
Eightupman

Reply to
Shep

Any one have any info, links, TSB's, ect. about these engines developing cam problems????? Seems like I remember someone saying something in this group about that. I'd like to know the how's and why's about it if possible. I recently went from using 10W30 to straight 30, only because the book said I could use 30 if 10W30 wasn't available. I always trusted straight weight over multigrade, esp. since this car is driven only in summertime and in the midwest, can get very hot. This 30W has been in it less than 1000 miles. I wouldn't think that could cause this anyway, but now I gotta wonder.

Reply to
James Goforth

Have you checked for a vaccum leak? On these engines, they were famous for sucking the intake gasket sideways. Because the vacuum leak is internal, you can't hear it. The best way to check is to remove the oil filler cap and spray something like brake cleen down inside the lifter valley area and see if there is any change. It may not be your problem, but I've seen this happen on many of the older 2.8/3.1 engines with the paper intake gasket.

Ian

Reply to
shiden_kai

yeah..you did...I was a little slow on the uptake....

Reply to
Eightupman

How old are the plug wires? I have seen similar problems due to bad wires.

Reply to
Scott Buchanan

Early on I had put the vacuum gauge on the engine and the needle was right in the proper range and steady, but I'll try looking for that internal vacuum leak, thanks. However, I just looked in the oil fill hole and it looks like it'd be difficult if not impossible to direct the spray near the #3 intake port. As such, I'm not sure I'd be able to determine if that's the problem--unless I shot something EXTREMELY flammable in there, like starting fluid. Would that totally raise hell inside the engine? Naturally, I'd change the oil immediately afterwards.

Reply to
James Goforth

Can you wiggle the wires in the main harness where the loom plugs in? Can you trace continuity of the wires controlling #3 injector? Just a WAG FWIW YMMV DFB

Reply to
MisterSkippy

Ian, I went ahead with spraying starting fluid into oil filler hole. First I disconnected the PCV from the rear valve cover and also the air tube from the front valve cover that runs into the flexible rubber intake duct. Sprayed into engine several times, noticable change in rpms. Question 1. I detected suction into the oil filler hole--is this normal, with the PCV disconnected? Or does it further point to the leaky intake gasket? Question 2. Could the rpm change be due to some reaction with the O2 sensor, causing an adjustment, or something like that? That's another reason I balked at using starting fluid. Note that my penetrating oil ("Deep Creep"--petroleum based, flammible penetrant from Sea Foam) didn't produce any reaction. Question 3. What causes that gasket to "move" out of place after being carefully torqued between two machined surfaces? (since I just put that intake on last summer with the new heads, & don't want to have the same thing happen again).

Never found any TSB's on this, BTW.

Reply to
James Goforth

No, there were never any TSB's on it that I recall. It wasn't something that happened as frequently as....say, the intake manifold gasket problem that is occuring these days with the 60 degree engines, but it happened often enough for it to stick in my mind as something worth considering.

Why the gasket moves?.....I don't really know. By the way, you don't have to worry about actually spraying right at the cylinder that has the miss, if you just spray into the engine, if the fluid is the correct kind, it will find it's way into the offending cylinder.

According to your diagnosis procedure, I'd say you have little to lose by checking the intake gasket. You've pretty much eliminated everything else. Remember, most problems boil down to "simple" things going wrong.

Ian

Reply to
shiden_kai

I removed the intake (not a lot of fun) and the intake gasket is one of those "valley pan" style gaskets, the whole thing is one big piece, made from that thick paper-like stuff. Thus impossible to get 'sucked sideways', perhaps the reason for this gasket design. But leaky intake is still what the symptoms suggest, not to mention process of elimination with all the other stuff I've done, LOL. Although I saw no evidence of leakage, I'm not sure it'd be visible anyway. I measured the compressed height of the valve springs as best I could with a vernier caliper (couldn't do it on the rockers as I don't have a dial indicator) and the valve lift seems to be very close (comparing adjacent cylinders).

Incidentally, the intake has the ports numbered 1,3,5 cast into the front (grill side) of it and 2,4,6 on the firewall side; my Haynes manual says 2,4,6 is on the front and 1,3,5 on the rear (firewall side). It also shows 10 bolts on the intake and there's only 8.

Those 2.8 intake manifolds are actually quite large and it seems like the fastening system (8 long, skinny bolts, not much torque) would be inherently somewhat deficient and lend itself to sealing problems, IMO. Not hard at all to see that regular gaskets would have a very good chance of moving out of place, just like Ian said.

Reply to
James Goforth

When the heads were replaced, were new bolts used or were the old bolts re-used? Are these replacement heads or were the ones on the engine re-newed? Did you have this problem before the heads were replaced??

What is the psi of the two adjacent cylinders?

An Air-Leak check would be a good diagnostic help.

Maybe you have a coolant leak into the gas-mixture......water/coolant in the gas mixture will cause that cylinder to run overly hot, which could account for the color of the plug being whitish/grayish. A correct heat-range plug should show toward a light-brown color.....if the gas you use doesn't have a high sulphur content (which is another whole-can of worms)

Anyway, just tryin' to help....I know this kinda problem can drive you nutz, lol.

Dave S(Texas)

Reply to
putt

Yeah, I used new torque-to-yield headbolts. The replacement heads were exchange ones from the machine shop, which performed perfectly all last summer and this one. I had overheated the car badly last summer which caused one valve guide to drop far enough to hold a valve open; upon disassembly I found several other guides almost to the same point and thus replaced both heads. My initial check before starting that procedure indicated 150 psi on all cyls except one which had zero. The dead cyl currently has

150 and I didn't bother to check any others--all I know is I pull the plugwire off and the rpms don't change. I, too, found the appearance of the plugs to be "not what I expected." I run 10% ethanol (gasohol) in it most of the time, but this last time I put premium in it just grasping for clues what might change the way it was running. This might account for the coating on plugs(?) which are otherwise fairly new. I'm usually a little leery of buying premium in any case since it doesn't sell anywhere near as quickly as the other so you don't know how old it is... plus the prices now probably make it so nobody buys it at all.
Reply to
James Goforth

Sorry, James.....I'm quite familiar with the gasket design. It's very possible for it to move sideways. You just have never seen one do it. The gasket you describe is the factory style gasket for those year. GM has alternated between individual gaskets for each head and one-piece gaskets for the intake. None of them have been that great.

Ian

Reply to
shiden_kai

I've been thinking about what I can do to prevent this from happening again and I guess I'll use some of that permatex sticky gasket stuff that looks like 3M trim adhesive. Also I guess I'll clean the intake bolt threads and put some loctite on them just in case they could possibly work loose. When you say the gasket moves 'sideways' I was ASSuming you mean laterally, as in left-right as you're looking into the engine compartment from the front. Seems like if that whole big thing had moved like that there'd be more than one dead cylinder. But without regard to that, I can't fathom what else could cause the rpms to increase when I sprayed starting fluid into the oil filler hole with pcv disconnected, so this will be a learning experience and I'm grateful for the help.

That's what's so great about this forum: Before this, and you had a problem with an engine--what were your options? "Well. I guess I could ask this one guy, he might know..." Now you got the whole world.

Reply to
James Goforth

I replaced the intake gasket and the dead cylinder now lives on.

In the course of checking for other things wrong in the beginning, I found the EGR valve to be stuck solid. My donor engine had one whose pintle moved freely, and I installed it and could see the diaphragm move on it as I revved the engine. But the engine ran lousy and had all the classic symptoms of a faulty EGR, (ran rough under light load, wanted to die at stop signs). Curiously, it ran fine with the old, stuck EGR, and so I disabled (unplugged vacuum from) this replacement one and now it runs perfectly normal again. Any ideas why that would be? Just curious. Through it all, BTW, never have had any EGR codes. Also, thanks for the input.

Reply to
James Goforth

Great! Hopefully it will stay fixed.

Dave S(Texas)

Reply to
putt

Dont know the history, but the vacuum line may be picking off ported vacuum instead of manifold vacuum..

Reply to
Eightupman

What book is this you speak of? I had a 1988 Z24 2.8 and GM recommended

5W30.
Reply to
Steve Mackie

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