This is not a 3rd Gen question but I really need some help with this...My father has a 1997 GMC Yukon with a 350 which seems to want to jump out of timing at certain times and we can not find the problem. We've had this truck in 5 different GM shops and everyone tells us there is no problem, except one shop replaced the camshaft sensor and the crankshaft sensor, saying they tested bad but it hasn't made a difference either. Here are the symptoms: while driving the truck will start to shudder and then it feels like the engine has lost timing then it will cut off. A load doesn't effect it (pulling a trailer), it's totally random, it's not speed related, it's not temp related. The tach will jump from 2k to 3k then down to 1k and back to 2k then 0. Operating temp is normal, fluid levels normal, only 56k miles on truck. It doesn't do it at any certain speed or drive time. It may cut off sometimes and it may not but will buck real hard. We've had it to 2 GM dealers and 3 other shops and no computer codes were stored and the dyno shows nothing. The shop manual is very vague and doesn't contain info on testing any sensors "take is to a qualified shop", any ideas, we really need some help since GM service centers don't have a clue...

 

sounds like a bad coil or ignition module

 

doesn't it have the opti-crap ignition?

 

Opti-craps were on cars only as far as I know.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
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what makes you think it's timing? hard to diagnose without feeling it but it sort of sounds like a tranny problem, maybe torque converter lock-up problem. Could also be ignition system breaking down. Try placing your foot on the brake in gear and loading up the engine and see if it cuts out the same. If so, check or replace the plug wires and look for corrosion at the coil. don't see too many plugwire problems on the L31's but the coils do have problems.

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Kyle Osterholt
Okarche, Oklahoma
ASE Master Certified
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73 bronco

 

Some interesting news: I put it in park and brought up the RPMs to peak around 6k and held there for around a minute and a black cloud of smoke poured out the exhaust. Plug wires are new and coil tests OK. I was saying before something was effecting the timing because of the stumble and the RPM gauge jumping around like it did and it felt like it was coming out of timing the way it bucked. Could this be ERG related?

 

Sounds like possibly a fuel pressure regulator problem. That black cloud of smoke is a good indication. I would remove the fitting from the upper intake for the brake booster (you will have to remove the air silencer) and look down inside the hole and look for fuel puddled up. Sometimes they only fail when hot w/ vacuum applied, wich may explain the intermittancy of the failure.

Also, opti-spark was only used on L99 and LT1 engines.

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[This message has been edited by GMTech (edited February 25, 2001).]

 

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