Idling issues...a little help?

I have a '96 XJ with almost 230k on the 4.0. Lately I've been having a problem idling, and it seems to be getting worse. It barely idles at all, I have to do a brake stand at stop lights and keep the RPMs above 1000 so it doesn't die. It sort of stumbles and sounds almost as if it's starving for fuel, but if I hit the gas it runs fine. Drives great down the highway. It idles slightly better when warm, but only slightly.

So far I have:
- Replaced the spark plugs
- Cleaned and tested every connection from the battery, alternator, and engine grounds.
- Removed the throttle body and cleaned all the sensors
- Filled up with more than one tank of gas from more than one station to rule that out

And nothing has changed a thing. On top of that I can't seem to locate my repair manual.:sure: Anyone have any other suggestions?! Maybe I need to replace my IAC, is there a way to test it first?

I am *this* close to ripping off all the wires and sensors and bolting on a carb. :smsoap:

it sounds like you have a vacuum leak. vacuum gauges are $20 and vacuum hose is about $1 a foot from Advance

ehall said:

it sounds like you have a vacuum leak. vacuum gauges are $20 and vacuum hose is about $1 a foot from Advance

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Hmmm...can you elaborate more on what I should be looking for with a vacuum gauge? A leak certainly wouldn't surprise me, with all my old hoses...

you said you replaced the plugs...does that mean you did wires/cap/rotor button?

Joshooha said:

you said you replaced the plugs...does that mean you did wires/cap/rotor button?

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No, I didn't replace them, though I inspected them pretty well. I replaced the whole ignition system only about 25k ago, I only replaced the plugs this time becuase I found the gap was about .050 and I thought I might as well just replace them instead of regapping them all.

Testing for a vacuum leak around the intake manifold, throttlebody and vacuum hoses can be accomplished with a spray can of throttlebody cleaner....
Spray a light mist around where gaskets are where the throttlebody mounts, the intake manifold attaches to the head and on the vacuum hoses....
When a leak is found.... the mist will be momentarily drawn in by the vacuum closing off the leak and smoothing out the idle....
Has the feel of your braking changed since the idle problem started? Could be that the vacuum booster diaphram has a leak.....

While you're at it, and if the other suggestions don't help, how about a compression test? I've seen where leaky valves will cause what you're describing. Like suggested earlier, get a vacuum gauge, and find an unused port on the intake manifold. A steady gauge may rule out valves, and a jumpy gauge may indicate leaky intakes. With this many miles, just about any engine malady can come into play. You are going to have to be systematic at knocking down possible causes, and none other than a valve job will be too expensive. Pray for a vacuum leak! Checking the torque on the manifold bolts would be a good place to start, possibly a fresh gasket. One more idea- has the car been to the dealer for the recall? They check the cat, and re-flash the PCM. I had my '96 done in May, and it does idle smoother, and seems to get better MPGs. Just a thought.

Wow said:

Hmmm...can you elaborate more on what I should be looking for with a vacuum gauge? A leak certainly wouldn't surprise me, with all my old hoses...

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This is what a vacuum tester looks like



It shows vacuum pressure in inches of mercury (Hg), ie the amount of negative pressure needed to move mercury through a tube X number of inches. Most four-stroke engines are supposed to put out 15-20 hg inches of pressure. If you have a leak somewhere the engine will succeed at sucking in outside air so the negative vacuum will be somewhat lower. However since the air is bypassing the throttle body and air filter there are many problems that can result, short and long term.

Two of the common symptoms of low vacuum pressure are uneven idle (the computer has to compensate for unexpected amounts of air in the intake by tweaking the idle air bypass valve and the fuel injectors) and weak brakes (the booster uses vacuum pressure for its magic).

To test for pressure, find one of the rubber vacuum lines (like the one going to the MAP sensor on the firewall behind the engine) and splice in a T-connector like the one shown in the fitting. Make sure its tight, then start the engine. Vacuum will be pretty low at first but should build to 15-20 hg inches after 15-20 seconds. If it doesn't start looking for the leak or just replace all of the hosing if it is all dry and cracked. Buy about five or six feet of vacuum hosing (not fuel or heater hoses, but actual vacuum hoses), make sure it is small enough to form a tight seal, and use dielectric silicon grease to lubricate and seal the connectors. It's pretty simple really.

Thanks for the info everyone, I'll try to get ahold of a vacuum gauge and see what happens. I've been wanting to try doing a leak-down test just for a checkup, I guess now would be a good time for that? I guess I should have done it while I was changing the plugs, LOL. The old girl is getting pretty old but I've been looking for a good cheap donor XJ to fix it up a little. A stroker might be in the future....:doh:

Another thing - my exhaust manifold is cracked, like most are, and has been for years. It's never bothered me before, just made it a little louder, but I have a flowmaster anyway. Could that be causing the idle issue?

Usualy a vacuum leak will cause a high idle situation on a computer controled engine as the computer would notice a lean situation & compensate for it with more fuel, causing a higher idle. I would still check the vacuum & cylinder pressures to rule out valve or blowby problems. I would also check fuel pressure at the rail checking for a bad fuel regulator or pump.