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Jerry, Master Mechanic
Category: GM
Satisfied Customers: 17220
Experience:  ASE Master tech, 30 years experience GM certified
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99 Chevy Suburban 4x4 1500 5.7l (350) Vortec original engine

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99 Chevy Suburban 4x4 1500 5.7l (350) Vortec original engine 153K miles started running rough - replaced plugs & wires (hadn't been tuned up in awhile) - plugs 4 and 6 showed some red residue - all others fine color - no change w/new plugs - took to local shop who I know/trust - new plugs and wires fine - scope shows miss below 1100 rpm - comp test results cylinder #1)145psi 2)170 3)175 4)145 5)170 6)125 7)165 8)175; checked for blown head gasket - not the cause - checked for intake manifold leak - nothing - shot some oil into cylinder 6, retest for comp - 140psi, so looks like both valves & rings on 6 - check engine for vacuum leak - not able to find anything - engine does have small coolant leak @rt. front corner of intake manifold - we just watch it/refill as needed - burns about 1 quart of oil/1000 miles - know the engine's tired, but I've had 350s go >240K miles - my guy recommends new engine - WHAT else might be the cause or should we check before new motor? Why low RPM miss ?

Hello and welcome to Just, Super mechanic here. About your vehicle.

The vortec engines have an inherant problem with the lilttle poppets on the ends of the injector tubes that will give you those symtoms.

There is a test that can normally be done with the right equipment, an injector leak down test. That will give you an idea of what each injector is injecting.

Before replacing the engine it may be to your best interest to have the injectors checked or cleaned by a GM dealership, as they will have the proper equipment to clean them right.

Here is some of what the service bulletin states.


Some customers may comment on rough idle after start-up, especially if the vehicle has sat overnight. These symptoms may be intermittent. The Service Engine Soon (SES) light may also be illuminated. Current misfire or history misfire codes may be detected with the Tech 2 scan tool.


A deposit build-up on the CSFI poppet valve ball and/or seat may cause the poppet ball to stick open or closed. In either case, the specific cylinder will be mis-fueled, resulting in a cylinder mis-fire condition.


A new injector unsticking and cleaning process has proven to be effective in restoring poppet valves to an "as new" condition. CSFI injector replacement should NOT be considered as a correction for this customer concern.

Customer: replied 8 years ago.

My mechanic said he noticed some black coloration on the #6 plug when he did the compression test - fuel possibly? he said, so I think you've got it - I know the injector leak down test requires more equipment than I have (I'm a good barnyard mechanic - my dad was a heavy equipment/diesel mechanic - learned a TON from him) - Looked up a few other web sites - seems #6 fuel leakage/excess is indicative of a injection system problem-bad regulator) -


Thinking of costs of injection leak down test vs. just buying new parts for the same money - read some about the MFI conversion - I have already replaced the in-tank fuel pump on this burb, so we can rule that out - Is a MFI conversion possible on the engine? VIN #GNFK16R5XG191945


What would the cost (rough) difference be between cleaning the injectors and the MFI conversion at your shop?


Have you had much success with just cleaning the injectors?


I read the service bulletin on cleaning vs. putting in a new MFI - I'm guessing with #6 cylinder as the problem - (possibly leaking fuel has taken out the rings?) - that If I have them cleaned I'd need a new fuel regulator AND cleaning and/or spider - Thinking I may just MFI it myself if it can be done by a competent (but not expert) mechanic -


Not trying to take work away from the GM shops but his one seems doable for me - your thoughts?

I know the MPI conversion will cost a lot as the parts are about $3000 aftermarket for it.
The cleaning normally goes for about $150 at the stealership around here, (I have had to send some there from the enigne shop)

However, the spider assembly you can get after market for about $300 and that would probably be the best way to go.
Jerry and other GM Specialists are ready to help you
Customer: replied 8 years ago.

A couple easy last ???? for you - will hit accept after posting this question - thanks for your help!


Been thinking if I was into the intake manifold to replace the spider I should just yank the whole intake manifold and fix the small coolant leak at the front as well - My mechanic said to "really watch" cleaning the intake/block & heads for gaskets/debris - something about GM not recommending any power tools? for the gasket removal when replacing an intake gasket - He said he'll only hand scrape them clean now - GM had an issue with contamination in an oil line taking out a rebuilt engine?


Is there a service bulletin on this? Ever heard of it before? Where could I get a copy of the bulletin?


Any other shop secrets/hints for cleaning the well hardened intake gaskets off of an engine with >150k miles? Other than the obvious: keep scraping? :)


Thanks again for your help.........