Car struggling while idling but fine when driving

[Mark Tebb]

Hi there, I have a Mercury Tracer, 1996. Recently the car is struggling when it is in idle. However when I am driving it it is fine. The engine whilst driving sounds great and I don’t have to put my foot on the pedal more than normal to keep it going. So I am intrigued as to what the problem might be since it’s only in idle or in park when the engine is struggling and often stalls. Your help would be much appreciated!

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[Mike]

That thing is getting old. Some hose or fitting on the vacuum system might have cracked open and is letting some unmeasured air into the intake. At idle the total volume the engine is ingesting is small, so a minor vacuum leak can let a large percentage of extra air in. The same vacuum leak will make little difference when driving because there is so much more air is sucked into the engine during any driving condition than there is at idle. That’s why you notice it stumbling at idle but not when driving.

If that were the case, I wouldn’t be surprised if a check engine light comes on for it being lean (P0171) at some point.

I’ve also run into worn spark plugs causing a rough idle but drive fine, even though that seem illogical by books smarts.

[Erin]

Was this a sudden thing? Has it gotten worse? Did you use real cheap generic gas at the last fill-up? SOMEtimes this can cause poor idle. One of those things that may require some diagnostics.

Here are some things to check, in order of ease –
Have autozone or someone pull the codes. Codes don’t really give too much info but it is good to have them to get SOME general direction. Is the check engine light even on?
Inspect all vacuum lines. Typically the leaky parts are gonna be where they connect to something or other stress points like if they are strapped to anything. Do not forget the vacuul hose to the brake booster and hoses to the cruise control IF there is a vacuum line attached.
Inspect electrical wiring and connections.

Make sure the air filter is not too nasty. Not likely the culprit but just to eliminate it. Inspect the intake hose and the connections.

Clean the throttle body. During which, open it to clean inside and out best you can, especially the plate. Clean where the idle air enters the intake.
IAC valve. Could try cleaning it. Unless you want to snag a cheap one from a junk yard, don’t just go replacing it right away.

Make sure the ignition wires are in good shape, including connectors. make sure the plugs are in good shape. check the posts on the distributor or coils to make sure they are not corroded. Maybe not likely cause if that were the case, it would run badly thru all RPM ranges.

Some of the above things are more “wishful thinking” type checks than realistic possibilities BUT at least they are easy and can be eliminated as culprits. Plus at least you can know you did those maintenance measures.

Like Fopeano said – we might be leaning mostly towards a vacuum leak. If the hoses are good, there may be an intake gasket leak. intake leaks start off as rough or otherwise unstable idle but tend to progress to where it runs bad all the time.

Doing intake gaskets is not a bad job. A little hassle but on a 4 cylinder, you can likely do the job if you have any level of mechanical knowledge.

[Nicholas Clark]

Me too on the vacuum leak. Use a cigarette or cigar and see if the smoke gets pulled into anywhere near gaskets and vacuum lines.

[Zachary]

Sounds like a vacuum leak. I had this same issue on my 1991 throttle body injected 305. I found a leak around my throttle body and a broken vacuum line. I rebuilt the throttle body,and replaced the line,and runs great now. It was the same as yours,stalling at idle,but you wouldn’t notice a thing while driving.

[Mark Tebb]

I took the car into a garage today to get a free diagnosis of the idle problem. They say it needs a new ‘PCV to elbow on top of throttle body and fuel pressure hose’. On top of this the car needs a new power steering pump (or I am wondering if you topping up the low fluid would make a difference or not). Anway the cost of all the repairs is probably as much as the car is worth. I have attached a copy of the repair order and cost. So just wondering what people think or advise re this.

Thanks guys, appreciate your help!

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[jonathan]

its a vacuum leak, when it idles or runs rough when stationary and runs good when driving its most certainly a vacuum leak at a gasket or at a vacuum hose, i would use carburator cleaner and test vacuum areas and spray it and if the idle rises you’ve pin pointed the problem area, i had that problem with my car i ended up replacing the intake manifold gasket.

[jonathan]

i would dry to do the repairs myself as for me, power steering fluid flush and power steering pump replacement arent difficult and you will not be charged labour time for them

[Author]

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