Extreme backfire; throttle body? TrailBlazer

Look at the vacuum lines and the MAP. The back fire could have split a vacuum line or damaged the IAC passage. That could give you an air leak that the IAC cannot control.

wrote: (RE: 2002 Chevy TrailBlazer backfire)

On occasion on cold start-up if the key is returned to the "run" position from the "start" position just as the engine seems to be running it will backfire into the intake. Usually not harmful; start it and everything's fine. Once while under warranty the backfire caused damage; the engine would run normally at speed but it surged violently at idle. Chevy dealer repaired it and told me the "Throttle body was bent". It backfired again this week, resulting in the same symptoms. Starts and idles fine for about a minute; then starts surging -- back and forth from about 800 rpm to 1200 rpm every second in neutral -- if in gear extreme shaking. Runs fine at speed, but won't idle down. The error code is: P0507 Idle Air Control System RPM higher than expected. I removed the throttle body but found no damage. Plate moves freely; spring loading on the plate seems to be what I'd expect. Not gunked up. What could it be causing this problem immediately after the backfire? __________________________________________________________________________

The root cause of the problem is probably a defective ignition switch which disconnects and quickly reconnects the ignition coil during the key release at starting, causing an out-of-time spark to ignite the fuel in the intake manifold.

The resulting high pressure pulse can bend throttle body butterflies, making it impossible for them to fully close and allow the IAC to do its job.

The pressure spike temporarily shoves the IAC and EGR pintles off of their seats. The EGR is pretty sturdy, but the IAC could be distorted and unable to be repositioned by the engine's computer.

You might replace the ignition switch first, then the throttle butterfly, then the IAC valve if that didn't help.

Good luck.

Rodan.

Rodan,

I really appreciate your suggestions and will follow up on all. You clearly know about this stuff.

Directly to the issues .. it idles and runs fine for maybe a minute when the engine is started from cold (this in 75 degree weather where I live). Begins this idle at about 1200 rpm; then drops purposely to about 800 and the surge is on! Then it continues to run fine at speed, but surges dramatically at idle. Would that not argue against a vacuum leak which should also cause the problem when the engine is cold?

The butterfly sure seems to me to move easily, smoothly and freely, with no evidence of damage. Push it gently open against whatever is giving it spring-like tension and it feels perfect with no binding or evidence of scraping. It does seem to close completely. (Plus for about a minute of running from cold everything seems normal.)

This fly-by-wire system has stuff in the throttle body that I cannot examine .. is there anything there that might have been damaged by the single backfire causing this problem??

Bottom line .. single big backfire on one single occasion immediately followed by huge surging idle issue after running for about a minute ; runs fine at speed all times.

Is there anything else other than the throttle body that would likely be damaged by this backfire that could cause these symptoms??

Need help, and thanks .....

Mike

It would depend on where the leak was. There are a couple lines that have thermo controls on them. Until the engine is warm they are closed off.

Steve,

Would these lines that have thermo controls be able to or likely to have been damaged by a single big backfire?? Because there really does seem to be a strong correlation between the two events.

Can you tell me where I'd look to find these or have any suggestions on how to diagnose??

Getting a little desparate her because I really need to use this vehicle over the holidays.

Thanks ....

Mike

You probably have too much air getting into the intake so that the idle air control can't reduce the idle (air flow) as much as it wants. There are 3 things (i.e. things that could have been caused by the backfire) I can think of that could be causing this:

1) The throttle plate may not be closing as much as it should. This might be diagnosed by inspection (if you know what to look for). It should have an external stop and internally it should be pretty close to cutting off the flow of air. When they close completely against the bore they usually stick a little. That is why there is an external stop that holds it so that it is at the point where it is almost shut tight. 2) There could be a vacuum leak somewhere. That means it is sucking air where it isn't supposed to suck air - such as a vacuum hose or gasket or crack in the intake manifold. 3) The idle air control is damaged or due to crud it can't shut down as much as it is programmed to. My guess is that it is programmed to back up and try again when it can't shut down the flow enough - that is what is causing the surging. When the engine is cold it needs more flow so it isn't a problem until it is time for it to try to shut down.

-jim

Any vacuum line could have been damaged. The pulse from a backfire can be very nasty. I have seen carbs broken in half from a bad one.

Look on the line routing sticker and follow the lines. A bad line isn't hard to find.

Is this throttle by wire?

Steve,

Yes .. this is throttle by wire.

Additional info: Today I took the throttle body out to the Chevy Dealer and spent probably 1/2 hour with the Service Writer, a select Technician, and Parts Guy -- all very experienced, knowledgeable, and seeming to try their best to help me. Took their time and really seemed to make an effort to think it through.

Nobody "saw" anything wrong with the throttle body; a search of their database on the computer came up with no information; and the "brainstorming" we did only covered ground that I'd already traveled. We all agreed that since a throttle body replacement was the apparent fix during the previous episode with exactly the same conditions and symptoms that would be the most likely fix again. They felt they could be more confident if they could actually get their hands on the car; and that I understand and agree with -- they were basing almost everything on my reporting.

But even if I wanted to try throwing parts at this by just replacing the throttle body myself I couldn't at the moment .. because they have none in stock and it will take at least two days to get one (which he quoted a price of $235 on).

I know the Dealer can do a whole lot more real-time diagnostics than I can .. so I think I've decided to take it in to them on Monday and let them take a look. Count on leaving it with them for a while.

Still betting that, for whatever reason even though it appears good -- that this will end up being the throttle body.

Still got time for anybody's further advice and suggestions ....

Mike

It seems you continue to look in the engine control systems and treat symptoms as opposed to the cause of the backfire. An intake backfire is a symptom of a valve sealing problem in many cases. Valve sealing is affected by seat condition, camshaft profile, valve spring condition and stem/duide condition. In pushrod engines, a bent pushrod can do this. Over the years, GM has had their share od camshaft wear problems and broken/weak springs are not terribly uncommon. Unlike a carbureted engine, EFI is pretty resistant to backfire caused by a lean mixture - that is where it lives. Think about it - there is no backfire without ignited fuel. Where is the ignition and how does it get out of there back to the throttle body? You may want to look for a vacuum leak between the intake and the cylinder head. If this is the problem, you should see a plug with indications of a very lean mixture in that cylinder since this is just past the injector. A vacuum leak would also explain the idle surge. I would expect there to be another throttle body replacement if you do not find and correct the problem.

Lugnut

That would be true, but he explained that this only happens on occasion when you let off the key when the engine is cranking. The theory is that the ignition cutting out and then back on produces out of time spark which could cause the fuel to ignite while an intake valve is open.

-jim

Jim's hit the nail on the head.

Plus .. my immediate problem is that the vehicle is unusable until I get whatever problem was caused for whatever reason that causes this uncontrollable idle surging.

Why is the idle surging .. fast and violent enough that it is like repeatedly hitting the brakes; hitting the accelerator wide open, etc. etc. after the engine runs aprox 30 seconds to one minute from cold ( 70 degree ambient where I live) This problem recurred immediately after the backfire; both times. So there must be some relationship .. I'd hope that description would lead to somebody figuring out what was damaged.

I presented the fact that this issue occurred about 60,000 miles and maybe 3-4 years ago and was apparently resolved with a new throttle body. Leading me to believe this might need to be the solution again .. but after examining the throttle body plate and seeing no damage to it now I don't know.

I appreciate all the concern I've received over the backfire issue (perfectly described by Jim) but that is not my issue at this moment.

Still need advice and help if there is anybody who might know .. and thanks.

Mike

Different car, a 2.0 Chevy. TBI. Surged intermittently for a couple weeks before the ECU failed completely. New ECU fixed it. And I was about to replace the TBI. Lucky. No backfiring, but the thing would rev to 3 grand. Hard on the brakes and nerves.

--Vic.