98 5.7 vortec misfire/compression mystery

[andfern97]

Newbie here. Just picked up (what I thought) was a great running, 1 owner k1500 with the 5.7 vortec and 140k on the clock (I even had a pre purchase inspection done). Ran great for about 2 weeks, super smooth on some longer fishing trips. I took it into an indy shop last week due to a small coolant leak, and they ended up replacing the intake manifold gasket. While at it, I had them replace the water pump (it looked old and corroded), plugs, wires, cap, rotor, thermostat, belt, and tensioner. Figured my truck'd run like a dream after all that work, but after driving it for a few miles away from the shop, I noticed it was misfiring on cyl4 (it didn't do that before all this work was done). Took it back to the shop that did the work, and they checked injectors and vacuum, but eventually found that cyl4 had low compression (100 dry, and 120 wet). They showed me a boroscope of cyl4, and it seems there was a lot of carbon buildup only in that cylinder. I figure since the wet/vs dry made a difference, it would be a ring, but at the same time how could a ring go bad overnight when the truck was running so well and all the other cyls are fine? The other possibility is a valve (they say some of the carbon could have gotten loose and plugged a valve overnight) but then why would the compression test be different wet vs dry? It's all very confusing to me I guess, maybe some of you veterans can shed some light on this mysterious issue... Any help appreciated. Thanks!
Original Poster
andfern97

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(Andy)
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Posts: 1
Topics: 1
Joined: 13 minutes ago
Location: Minnesota
Year: 1998
Make/Model: Chevrolet Silverado
Extra Info: 5.7 Vortec
Group Membership:
• Registered users

 

[tinfoil_hat]

Well, a few thoughts... #4 is on the passenger side and that's the valve cover you usually take off when doing the LIM gasket. The opportunity to screw up a valve is certainly there.... They also had to pull the distributer and they may not have set it properly on reassembly.

 

[andfern97]

Thanks for the reply. Yeah, my first thought was the distributor, but that would't affect cylinder compression though... They replaced the injector to see if that was an issue but it did nothing. Strange to me that just overnight once I got it back it ran like crap. I also got a code for an 02 sensor on pass. side which may cause rough idle, but again I don't know how that would affect only cyl4 and cause low comp...

 

[skylark]

When you replace the spider injection you have to pull it out of manifold. I could easily see carbon dropping down inside the valve. I would think by now that it would have blown out though. if it held the valve long enough to where the piston came up and made contact with a valve I could see a problem.

 

[andfern97]

That's what I would think. Apparently the valve looks good through the boroscope. Another strange thing I noticed was Im getting a p0507 high idle speed code and I can feel vacuum through my pcv hose now which would indicate the intake gaskets are bad.

 

[tinfoil_hat]

If you aren't losing coolant then maybe it's the upper leaking, not the lower. That would be easy enough to address. Chase all he vac lines. When I did mine I found the hard line that runs across the top was cracked, the small hoses were rotten, and a little elbow was bad as well.
then there's the bolts. The repair manuals say you gotta seal the bolts but I have had mechanics just look at me funny when I asked what they used to seal them.

 

[andfern97]

Yeah, I've never heard of sealing the bolts, so that may be an issue. I picked up a can of carb cleaner and sprayed all around the upper and lower manifold, but no change in idle speed. I removed the intake and found some oil in the tubing behind the MAF which was strange to me as well. This just keeps getting better and better!

 

[Fuzzy]

Lower Intake Bolts are supposed to by sealed as well as the Upper Left on the Water Pump (Better to just do all anyway) as they go into water jackets. Sounds like they messed up the timing pretty bad when they put the distributor back in and didn't care enough to make sure it was correct. O2 Code is probably unrelated unless its an Open or Short Code.

The compression test of 100/120 would be from a bad ring, and its probably not contributing much to your high idle and other problems. If they suspected a valve issue they could've done a leak down test and answered that pretty quickly.

You can try cleaning the MAF on your truck (Be careful, its delicate) which might help a little, but overall it sounds like the shop screwed up the intake job and is avoiding responsibility for it. If your truck is running worse after the repairs then they need to fix it.

 

[tinfoil_hat]

I removed the intake and found some oil in the tubing behind the MAF which was strange to me as well. This just keeps getting better and better!

I wonder if that doesn't point to a bad valve, though. You have that breather in the valve cover to the intake. Once again that's the same side as cylinder #4. I'm just wondering if there is a piece of old gasket or something that fell down there and is interfering with the valve or just straight up damaged it.

 

[andfern97]

Thanks for the replies guys. I cleaned the MAF and the rest of the intake tract well and ran some seafoam through a vacuum hose. Neither really affected anything. I got a can of carb cleaner and sprayed all around the intake gasket, the manifold, and everywhere I could think of and nothing affected the idle. This whole situation is driving me nuts. The worst thing is that the shop can't even seem to figure out what is going on short of a full rebuild.