Mechanics

ROUGH IDELING

2005 Chevrolet Venture • 174,000 miles

Hi, my engine is running very rough. It started out idling a little rough and then it all of a sudden got really rough. It does not shut of but it is burning a lot more gas then usual. There is no check engine light on either.

thanks
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Fruitfly
June 27, 2013.



This could be multiple things. First it could just be bad gas, so depending on how much you have in your tank I would assume this is the reason. If it is anything else, the Check engine light should be on. Let me know about the gas and then we can go from there.

Seangrant00
Jun 27, 2013.
First have the diagnostic fault codes read. See if any indicate a misfire. That's what is causing the rough running.

Caradiodoc
Jun 27, 2013.
Hi,

thanks for your response, but there is no check engine light on and the gas is not an issue

Tiny
Fruitfly
Jun 28, 2013.
The Check Engine light is irrelevant. There's well over a thousand potential fault codes and only about half of them turn the light on. The light must turn on when the problem detected could adversely affect emissions. Obviously a misfire falls in that category, but a lot of things can cause a misfire and the Engine Computer may have detected a problem that could be related. You provided some basic symptoms but no test results or anything else we can use to figure out where to start. We have no way of knowing if a cylinder is losing spark, an injector is sticking open or closed, fuel pressure is too low or too high, or things like that. In the absence of any usable information, the fault codes are the best place to start. If indeed there are no related codes, that in itself is a clue because we will know the cause is something that is not monitored by the computer. Mainly that is the fuel supply system.

Given that a misfire developed and quickly became worse, bad gas is the most likely cause. I had two cars come into the shop in two days, and neither would start shortly after filling with gas from the same station. After a lot of testing and head scratching we threw a sample of the gas on the ground and threw a lit match on it. The fuel put the match out. Both engines started after a drain and refill.

That fact that you know it's using more gas than normal suggests you've been driving it like this long enough to notice. Too much raw gas in the exhaust system can overheat the catalytic converter and melt the catalyst. That will turn what might be a minor problem into an expensive one.

Besides fuel and spark, each cylinder needs compression to produce power. Burned valves and worn camshaft lobes could cause rough running but those show up gradually and don't suddenly get worse. If no other cause is found, a compression test or cylinder leakage test would determine each cylinders' condition.

Caradiodoc
Jun 28, 2013.
Hi,

Thanks again for the info. What about the MAF?

Thanks

Tiny
Fruitfly
Jun 28, 2013.
With it running as rough as you are saying, I would still hook a scanner up to to double check any codes. There are a few things that may result without the light, fuel pressure is one. Have you had the fuel filter changed out? Maybe even a relay for the pump or something related.

Sean

Seangrant00
Jun 28, 2013.