Got me stumped

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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.
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Have you done a compression check on #3 yet?

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

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You may have compression, but a worn cam lobe or anything that prevents proper valve actuation can cause a power loss, verufy proper valve openings by measuring the valve lift and comapring it to an adjacent cylinder.

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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.
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What book is this you speak of? I had a 1988 Z24 2.8 and GM recommended 5W30.
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That book I referred to was just a reference book from a Sears Auto Center. I'm sure they get their oil recommendations from somewhere reputable since there are liability issues if they used an incompatible oil. I don't remember what other weights it recommended but I just remember it said that straight 30 was acceptable in the absence of multi-grade (only in warm climates, obviously, which mine is). I'd read in a Hot Rod mag years ago that straight weight afforded superior protection versus multi-grade, i.e. 30W versus 10W30. So I always used it in my '67 Buick GS400 convertible, (only driven in summer also).
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James Goforth wrote:

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
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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.
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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.
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James Goforth wrote:

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
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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.
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James Goforth wrote:

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
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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.
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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.
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Great! Hopefully it will stay fixed.
Dave S(Texas)
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Dont know the history, but the vacuum line may be picking off ported vacuum instead of manifold vacuum..

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

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On Wed, 31 Aug 2005 19:59:28 -0500, snipped-for-privacy@webtv.net (James Goforth) wrote:

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
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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)
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