Frank Williams, former TTAC Managing Editor and resident snark-whiz, writes:
Oh Great Guru of the Gearbox, perhaps you can honor me with your wise words of wisdom concerning my ’99 Chevy Tahoe. It has the 5.7L engine and about 123K on the clock. At speeds above 50 mph, it surges when accelerating. This happens whether my foot is on the pedal or the cruise control is engaged. It’s almost like someone’s goosing the throttle and you can see the tach jump back and forth over a 2-400 RPM range when it happens. I use a Tier 1 gas in it and I’ve tried the easy stuff like fuel treatment, fuel injector cleaner and cleaning the MAS. So what should I look for now?
Sajeev answers:
Have you considered replacing yours with the one above? If you hunt around, I’m sure you can get a good price. I kid! I kid!
It sounds like a vacuum leak to me. At least in theory: the leak is bad enough for the engine computer to compensate at low RPMs, hence the throttle spikes you experience on the road.
I’d start your diagnosis at the rubber lines on the (plastic) intake ductwork. Then remove the duct and look at all the rubber hoses at the intake manifold. Follow all of the lines to their next ending point. You are checking for spongy, cracked or brittle rubber lines. Replace any line that you even remotely suspect, because rubber isn’t made to last much longer than 10 years and 100k. And that’s me being generous with times and mileage.
If that doesn’t work, I’d still suspect a leak. Check at each intake gasket and vacuum line with an (unlit) propane torch and see if the idle spikes in a certain test area.
WD40 works too for detecting leaks. The idle air valves on engines makes vacuum gauges useless at idle and it also compensates for vacuum leaks so everything seems fine when idling.
Replace the spark plugs and spark plug wires before moving to anything more complicated.
I’ve had this happen on two of my vehicles – on a Honda motorcycle (plugs were to blame) and a 2002 Subaru Legacy (wires were toast at 90 000 kms). It’s extremely irritating.
A ’99 5.7 is, I believe, a SB-II smallblock and probably has an EGR valve. If the valve is carboned up, it will open (or not open) at an inappropriate time which could account for the surging.
I’d also download any OBD-II codes that the ECM may contain. It is possible for codes to be present without the “Check Engine” light illuminating. This could help point you in the right direction.
Generally, if you have a vacuum leak that accounts for a 200-400 rpm swing, you’d be able to hear it at idle when the vacuum is usually the strongest.
rpol35 :
I’d also download any OBD-II codes that the ECM may contain.
The first thing I did was check the codes. Nothing there.
I’ll check for vacuum leaks then move on to plugs and wires if that doesn’t work. Thanks for the tips!
Torque converter locking/ unlocking? Just throwing it out there.
Things that I would suspect on a Vortec 350:
1) Plugs, wires, cap, and rotor. The distributors on these engines are known for trapping ozone in the cap, which corrodes the “contacts”.
2) EGR valve (as mentioned above). You might be able to get away with cleaning the pintle and seat of the valve, and installing a new gasket that has a screen to help minimize carbon build-up in the future.
3) Fuel pump or fuel pressure regulator. Begin the diagnosis by checking the fuel pressure under a variety of driving conditions; you should see something in the neighborhood of 55 PSI. If the fuel pressure sags under WOT, then it’s probably time for a new pump. If the pressure generally seems high enough with the engine running, shut off the key and watch the fuel pressure for the next several minutes. If the gauge drops by any appreciable amount, the problem is likely to be the fuel pressure regulator.
4) These engines are known for leaking intake gaskets, although this usually first manifests itself as a coolant leak. If the gaskets are deteriorated enough, though, it could cause an air leak into one or more intake ports, which then might manifest itself as a lean surge.
5) There may be an issue with the torque converter lock-up clutch (TCC). It could be something minor like the brake pedal switch, it might be something internal to the trans but relatively easy to repair (like a worn bore for the TCC PWM solenoid, which is a common problem on the 4L60E transmission), or it might be a more significant transmission issue. The TCC will unlock with slight pressure on the brake pedal, so try duplicating the issue, and then *lightly* ride the brake pedal to disengage the TCC and see if the problem goes away (the revs should rise by a few hundred RPM as the clutch unlocks and then stay steady). Obviously, you can’t perform this test with the cruise engaged.
Good luck!
Try to replace the ignition spark plug cables. ti think that would be it. i had it on one of my cars… tried every sort of soft solution with gasoline additives, then solved it…
If I had to start somewhere, like someone else said, vacuum tube leaks then the intakes. Check for oil in antifreeze/ antifreeze in oil. Easiest way to check for oil in antifreeze is taking off radiator cap and looking for brown sludgy goodness.
Do much towing and/or neutral drops? Torque converter would be next usual suspect. Actually, it might be first….
That front platform is just crying out for a gun rack and kegerator.
Just sayin’
–chuck
EGR may or may not throw codes depending on whether your engine has a sensor that tells it whether EGR is open or not. This being a 99… odds are it doesn’t.
EGR most likely fits the description you states. If it’s stuck closed, and the ecm thinks it’s on, it would be compensating for something that’s not happening. Definitely check for vacuum leaks though and it could be those stupid intake gaskets. I don’t get how they could have screwed up something as simple as an intake gasket.
When was the last time the fuel filter was changed?
Vac problems can manifest after the engine bay gets hot and hoses swell, even if it ran fine when it was first started and cold.
Eric Bryant listed them in the right order. My guess is that it’s the fuel pump, but the other things are cheaper and easier to replace/check first.
How much is it to get in the pool? Put me down for fuel pump.
I think the distributor on your model is located behind the harmonic balancer and is a bear to service.(Incredibly poor placement!) Seems to me it is called a “crab”. I’m not sure if the wires detach from the head unit or not. You may need a tech manual or some on-line help with the ignition layout. Sorry!
@rpol35 – you’re thinking of the GenII LT1 and its “Optispark” distributor. The GM trucks from this era (GMT400 platform) got a GenI “Vortec” engine, with the distributor in the usual area at the rear of the intake manifold. The distributor has a bit different layout than the traditional HEI, but it’s still easy to service – if you don’t mind laying across the top of the engine to reach it…
Eric Bryant :
@rpol35 – you’re thinking of the GenII LT1 and its “Optispark” distributor.
Eric – Thanks for keeping me straight!
Frank – Nevermind!
Is there going to be a “Stump the Chumps…er, B&B” follow up?
yeah, whatever it is , please post a follow up.