Vortec 350 missing

[Alex_V]

And not found under hood after brief search... bummer.

Trying to fix a consistent misfire in a stock '97 GMC K1500 on #5 cylinder. 182,000 miles, shows a hair over 180 PSI compression on that cylinder when the others in that bank are exactly 195. Plugs and wires were done recently, and the fouling on that plug was dusty, soft, white buildup - swapping in a new plug didn't do anything for it, even temporarily. I pulled the plenum off and performed the injector balance test via my Snap/on "brick" graphing scanner and a fuel pressure gauge - #5 behaves just like all the others. I cleaned the intake, externally cleaned the injectors and spider, and replaced the distributor cap & rotor which were absolutely toast, but the miss is still there.

Initially the owner said it needed intake gaskets - that it was burning antifreeze on no.5. But by my troubleshooting and the fact that there are no coolant passages anywhere near #5's port from intake to head, I suspect the white buildup was just ash and such from that cylinder not firing at all, and lower compression was due to that cylinder being cool and having junk built up from no combustion taking place. The tip of that injector has a dark redish-brown tarnish while all the others were bright and clean - suspected the injector was bad as soon as I noticed that but it worked in the test so I'm puzzled. Is it likely the injector worked during the test but not under running conditions? The owner is open to replacing the injector spider assy. if need be but neither of us want to throw parts at it. Only code stored when I brought it in was 0300 Random misfire. Any help?

 

[bmod305]

Check fuel pump pressure. Vortec's need exact within spec pressure for the spider system to work. I know it 5 with a miss but I have seen weird stuff with low pump pressure even by 2 psi

Dave

 

[Ramblejam]

So you didn't replace the intake gaskets? Even without water/coolant intrusion, it can still suck air and oil from the valley.

 

[JimPghPA]

You could swap the injector from no.5 with one from a different cylinder and see if the miss then goes on at the different cylinder that got the injector that use to be on no.5

 

[Alex_V]

I didn't do the gaskets yet - wanted to put it all back together with the new cap and rotor to rule that out because nothing else was obvious. Is there any way to zero in on a vacuum leak before taking it apart?

And fuel pressure was a consistent 56 PSI.

 

[Alex_V]

Swapping the injector may be my next approach.

 

[Scramblerguy]

If it has the old style plastic intake gaskets it could have a small crack and suck oil out of the valley. Happend to me on my Vortex. Also on some Vortex engines I think there was a service bulletin for valve guides that were too tight and would cause the exhaust valve to stick and cause a miss and low compression not that yours is that low. This also happened to me. This is all just a guess....but I hope it helps.

 

[Alex_V]

Any suggestions or common ailments are welcome, scramblerguy. I've maintained and driven several of the Vortec 5.7's but this is only the second one I've done any work on, the first being 4-5 years ago before I was nearly as savvy to newer induction and computer systems as I am now.

 

[FlyNavyP3]

Alex,

I hate to be the bearer of bad news, but it may be coolant intrusion due to a cracked head in the exhaust seat area. Unfortunately not completely uncommon on the Siamese exhaust ports (3,5/4,6) on the Vortec engines. Is it loosing coolant?

I'd investigate other options first, but it's possible that the problem is deeper.

 

[Ramblejam]

Originally Posted By: Alex_V
newer induction and computer systems

You think of the long defunct CSFI system as "newer"?

...looks down to a '67 Burb/'85 3500...

Oh, I see.

Regardless, replacing the intake gaskets and converting over to MPFI has vastly improved many an old 5.7L

 

[Trav]

It sounds like it may have a bad poppet valve on the spider. These things get unreliable as all get out as they age and are not worth rebuilding, the best thing to do with these is to install a MPFI Spider conversion, its a simple plug and play deal.
Did you use a sniffer for combustion gasses over the open radiator cap with the engine running or a cooling system pressure test? I have seen head gaskets leak into 1 cylinder only, at first its very small but it does over pressurize the cooling system usually showing up first as an overfilled overflow bottle that goes down when the engine cools.
As the other poster said a crack in the head isn't unknown either.

 

[Scramblerguy]

I'm wondering if the color on #5 injector is from oil from a leaking intake manifold gasket. You probably already know this but Felpro makes a upgraded gasket that is rubber coated steel instead of plastic. I've seen where someone installed the plastic gasket incorrect and the gasket moved down a little when they set the manifold in place and after a short time it leaked oil/coolant. The upgraded spider assembly does improve driveability and like others said is a plug and play. Be careful of the pedals that surround the old injectors they get very fragile. Just more ideas off the top of my head. Good luck.

 

[nthach]

The CPI spiders in those tend to go out like Trav said - leading to random misfires. I care for one in a friend's truck, gave him a heads up if I catch a P0300 code.

If you're going to replace the LIM gaskets on those, use the Fel-Pro PermaDryPlus steel/polymer ones or their equivalents only. It's too painful of a job to cheap out here(pulling the manifold and CPI assembly is easy, disconnecting stuff and retiming it to avoid a P1345 cam/crank code isn't). The cap and rotors on those tend to trap moisture and corrode the Al terminals in these, use the OEM AC Delco/Delphi caps in these if possible, Standard's non-economy line seems to be working for me.

 

[Alex_V]

Turned out to be the intake gaskets, after all. After watching the misfire go away completely above 1000 RPM on my scan tool and finding that plugging the PCV (which had stupid strong vacuum - felt like manifold pressure) with my hand would also stop the miss, I was sure enough that I tore it down - found the bottom half of 5/7's gasket leaking along pretty much the whole length, and some questionable spots on other cylinders, too. Purred like a kitten when I took it back to the owner.

 

[Scramblerguy]

Glad it worked out and thanks for letting us know what fixed it.