98 Chevy 5.7 Cylinder 5 misfire

The intake gasket and valve cover gaskets are not the bad to do. You will spend more time cleaning the gasket surfaces than turning wrenches. I have the exact same diagnosis for my 98 5.7L Tahoe. The dealer told me the number #5 cylinder was mis-firing and it had a bad O2 sensor. They told me to replace the #5 fuel injector and replace the bank 2 sensor 2 O2 sensor. I replaced the O2 sensor and the #5 fuel injector. No improvement in the Tahoe's mis-firing problem. In the past year I have replaced a bunch of parts trying to correct this issue. Plugs, wires, cap, rotor, lower intake gasket, upper intake gasket, fuel filter, had the injectors cleaned, and more. This is starting to sound like a lot of B.S. Why is it that no one can make a proper diagnosis for this?

I wouldn't say " no one" can diagnose it! What's the fuel pressure is it high enough? too high? bleeding off? The lines to the injectors on those engines tend to leak, especially after they've been worked on! Has a compression test been done?

in dealing with my 5.7, the engine was abused with no oil changes for many miles. bought the truck 2nd hand and this is the second engine, so I was told. 16 years of working on marine engines has taught me that at idle (ie. city driving) if the engine uses oil the (valve gaskets or umbrella's) need replacing causing oil deposits on the plugs. if under load (ie. highway driving) if the engine uses oil the (wiper rings and/or pistion rings) are stuck with black carbon. diesel oil with the same weight parameters for the auto oil has a very high detergent to help unstick wiper ring (5 oil and filter changes have finally cleaned the wiper rings) in the engine of mine. As for the umbrella's (short of doing it your self or removing the heads and having them rebuilt) is the only answer. over heating and old age caused my umbrella's to crumble in my hands during replacement. The new problem on the market is goo buildup from (ETHANOL) 10% to 40% currently being put in fuel; now-a-days this causes plugs, valves and injectors to foul or be detroyed. may or maynot be related to auto - fiberglass tanks with ETHANOL will cause the catalyst from the resin to mix with the ETHANOL and produce a jell in the tanks, once in the fuel line and passes throught the filter to the engine and burned; when cooled it will solidify on valves and passages. what a mess and expense to non-ethanol engines. along with ethanol related issues the corn properties collect moisture (ie H2O) and carries this to the sensors and such destroying them. new filters are being produced (very large) to eliminate the water/gel from entering the engine system. very new and sales are up. wonder why.....ha ha

hadn't heard that about ethanol but I suppose it's quite possible!

Hey so I'm experiencing the same problem with my truck. 98 gmc 1500 5.7L I'm curious if you were able to find and fix the problem. I have replaced my valve seals anticipating that was the problem, but it did not fix it. My next thought would be the intake manifold but I'm not really looking forward to throwing money at things that won't take care of it. My issues are identical to yours other then I have noticed very little ash burning on cylinder 7, not 3.

Spyder_Man start a NEW TOPIC post all codes - describe issue.

I just went through the same thing as you all did except mine was a #3 misfire on a 97 5.7 it is going to be the lower intake is letting just a little bit of water in the cyl. And yes you can replace lower without putting upper, just be sure and don't get cheap on the intake gaskets buy the sure cures or some other rubber sealed gasket there are several out there.

It seems you have proven that the intake gasket is leaking, but given the trouble I've had with these CSFI spider injectors (if you plan on keeping the truck) I would certainly spend the extra $300 to do the conversion if I had to pull intake anyway. These things will leave you stranded. Maybe do the job yourself and just spend the money on parts with new injectors. Been a couple of years since I did one of these, but I think I had to pull one or both valve covers to get intake off cleanly without fighting it.

I just did one of these , have a high idle now that settles down , so must still have a gasket leak. But you do not have to pull valve cover for the upper intake on a 1998 5.7 chevy. if you remove the lower intake then yes, one side at least both side makes it easier. now just have figure oure out why hi idle. sprayed with carb cleaner and start fluid i couldn't tell a difference . I will try removing pvc and other tube on passenger side see if there is pressure or vacuum there I guess. P0507 code.

I have same misfire on number 5 on a 96 GMC 5.7. has compression and spark, checked all the basics, number 5 plug is black so I did change the plugs and cap and found a hole in the pvc line and fixed it. Didn't fix my problem.

abouttime, most posters do not return to post a fixed or not post.. we give ideas based on experience* sometimes there is a follow up often not... start a new topic- give as much info as you can someone will try to help with your issue * in this case the spider assy is a known problem and a first suspect after the basics.

Girlfriend has a 98 chevy with the 5.7L and a intermitent misifre on cylinder 5 and dead miss on 7. I have changed plugs, wires, cap and rotor, upper intake gasket and while in there did the conversion on the fuel injection unit. It is still dumping fuel. No change. Noticed the truck uses some water from time to time. No water in the oil, but the exhaust is seems a little steamy. Only thought is lower intake at this point. Has anyone confirmed this thesis? Austin

have you done compression test, checked for vacuum leaks....how does #7 plug look compared to others...

My brother had the same problem. Small town mechanic took the engine apart and took pistons out and found crack in cylinder wall! It started overheating.