High idle (no engine code)

[MuffinMan1234]

1997 TJ 2.5L. I’ve replaced the IAC and the crank shaft sensor (not sure if that deals with the high idle). I have no engine codes and I have driven nearly 50 miles after replacing both. I’m not very knowledgeable about cars yet but I’m still learning.

 

[InOmaha]

Have you tried cleaning the throttle body and checking for vacuum leaks.

 

[Paparock1]

as stated above, check your vacuum lines, there are three at the base of the throttle body. Those lines there were really soft and not sealing. follow one of those lines to the front of the valve cover, that fitting was broke on mine.

I just had this happen to my 1997 2.5 and still chasing down the issue. Now that I have the vacuum leaks fixed I am testing the TPS and IAC. My TJ will run good when cold but when I drive and it gets up to temp the idle will not come down below 2000 when at a stop light. Then if I shut it off and try to start it again without it cooling down it is very rough to start and backfires through the intake and spits and sputters until I rev the RPMS up a few times.

Fun stuff. I will post here what I find so maybe we help each other out.

 

[MuffinMan1234]

as stated above, check your vacuum lines, there are three at the base of the throttle body. Those lines there were really soft and not sealing. follow one of those lines to the front of the valve cover, that fitting was broke on mine.

I just had this happen to my 1997 2.5 and still chasing down the issue. Now that I have the vacuum leaks fixed I am testing the TPS and IAC. My TJ will run good when cold but when I drive and it gets up to temp the idle will not come down below 2000 when at a stop light. Then if I shut it off and try to start it again without it cooling down it is very rough to start and backfires through the intake and spits and sputters until I rev the RPMS up a few times.

Fun stuff. I will post here what I find so maybe we help each other out.

Where did you get your vacuum lines from?

 

[TJ4Jim]

Where did you get your vacuum lines from?

Look at the auto parts store for generic rubber vacuum fittings and plastic hose. There is nothing special about the fittings.

When you inspect you vacuum lines use you hand to confirm the fittings have not hardened or have begun to crumble.

 

[D M]

My buddy had a problem with high idle, he replaced the spark plugs and that fixed it for him.

 

[MuffinMan1234]

Look at the auto parts store for generic rubber vacuum fittings and plastic hose. There is nothing special about the fittings.

When you inspect you vacuum lines use you hand to confirm the fittings have not hardened or have begun to crumble.

What size are they and if I were to replace all the lines how much feet is that?

 

[TJ4Jim]

What size are they and if I were to replace all the lines how much feet is that?

I usually just take the fittings with me to match them up, they just need to be close as they stretch a bit, and there is not much more than 3 feet of vacuum tube under the hood.

 

[Jerry Bransford]

My buddy had a problem with high idle, he replaced the spark plugs and that fixed it for him.

If there's a problem new spark plugs won't fix it'd be a high idle. A high idle is nearly always caused by a vacuum leak.

What size are they and if I were to replace all the lines how much feet is that?

You don't fix a high idle by replacing all the vacuum lines, especially since not all are easily accessible, you fix it by figuring out where the leak is coming from. The easiest/best way I know to locate a vacuum leak is to first start the engine and then spray all the vacuum lines and fittings with an aerosol spray like Throttle Body or Carburetor Cleaner. When the spray gets sucked in through the leak the engine rpms will stumble.

Here's a good illustration showing the visible vacuum lines. Item 8 is located under the battery shelf, it is the vacuum reservoir which stores vacuum for the times the engine can't generate enough.

One more vacuum hose not shown on this drawing is a large diameter rubber hose that connects between the intake manifold and the power brake booster on the driver's side of the fire wall, that big black round cannister. That vacuum line is what powers your power brake system.

 

[D M]

If there's a problem new spark plugs won't fix it'd be a high idle. A high idle is nearly always caused by a vacuum leak.

You don't fix a high idle by replacing all the vacuum lines, especially since not all are easily accessible, you fix it by figuring out where the leak is coming from. The easiest/best way I know to locate a vacuum leak is to first start the engine and then spray all the vacuum lines and fittings with an aerosol spray like Throttle Body or Carburetor Cleaner. When the spray gets sucked in through the leak the engine rpms will stumble.

Here's a good illustration showing the visible vacuum lines. Item 8 is located under the battery shelf, it is the vacuum reservoir which stores vacuum for the times the engine can't generate enough.

One more vacuum hose not shown on this drawing is a large diameter rubber hose that connects between the intake manifold and the power brake booster on the driver's side of the fire wall, that big black round cannister. That vacuum line is what powers your power brake system.

View attachment 121553

Interesting, i don't know much about that stuff, all he told me is that he replaced spark plugs and was fine

 

[InOmaha]

You can also get a vacuum leak in the throttle body gasket. Spray the carb cleaner around the base to see.

 

[Paparock1]

Where did you get your vacuum lines from?

Sorry for not responding sooner, I work nights so I am normally on here in the morning only.

I actually used F4 tape (do a google search) on all the fittings. this tape sticks to itself, its silicon so you can stretch it tight. You can wrap up fittings very tight to seal up air leaks just to get by until you figure out where your leak is at. I also used a old piece of vacuum line and some hose clamps on the valve cover to seal it up.

The F4 tape should probably be in everyone's go gear bag as it is very versatile. (IMO)

I found the leaks by spraying the carb cleaner at the lines while the engine was running just as Jerry described.

The majority of the vacuum line length on my TJ is hard plastic. Go to any auto parts store with a piece of your vacuum line you want to replace and they will get exactly what you have. If a hard line is broke I would replace it with just a rubber piece myself but you I am sure you could do some googling and find the exact replacement. I need to restock my supply so I was going to get about 2 ft of 3/8, 5/16, and 7/16 size lines.

 

[Evan77]

dylan, it sounds like a vaccum leak to me. Mine had it and i had to go get it fixed because i was tired of looking for the leak. Check all of your vaccum lines. Even a leak the size of a pin hole will cause problems.