idling and rpm problem

**Hogsters** · 12-06-2015, 12:41 PM

Just recently my ls1 started idling differently with the rpms fluctuating up and down. Everything was fine last time I had started it so Im not sure what happened. I've replaced the plugs and the inline fuel filter... there was no change after replacing either one. At some point it just dies out, but I can easily start it back up. Anybody have an idea what might be going on? I not to familiar with these motors and it sits im my 98 Tj. Came out of a 1998 Firebird.

**cammed goat** · 12-06-2015, 02:18 PM

Could be a number of things. A vacuum leak could be the culprit. I would start by checking the intake tube between the airbox and throttle body as that could be loose/cracked, check your idle air control valve(IAC) as those become dirty over time. Also check and clean your mass airflow sensor between the airbox and intake tube as they become dirty. Clean that with some CRC MAF cleaner. Could also be an EGR issue as those tend to crack over time. If you need anything else, give a shout.

**35th-ANV-SS** · 12-06-2015, 03:00 PM

Any codes?

**Hogsters** · 12-06-2015, 04:09 PM

Thanks for the feedback! I'll be trying these one by one and step by step along with a few youtube video's on "how to". Hopefully I can find a diagram to show me the location of the parts somewhere online.
I tried running my OBD11 but not able to bring up any codes, I'm guessing because it was an engine swap to the jeep tj, I really don't know.
I'll post back with results hopefully this week.

**35th-ANV-SS** · 12-06-2015, 06:20 PM

I am just going to guess a vacuum leak somewhere...like a hole or crack in one of the vacuum lines. That's the #1 cause of rev's jumping around. Cammed goat's recommendations are also good ones. Let us know what you find.

**Hogsters** · 12-06-2015, 06:34 PM

Originally Posted by 35th-ANV-SS

I am just going to guess a vacuum leak somewhere...like a hole or crack in one of the vacuum lines. That's the #1 cause of rev's jumping around. Cammed goat's recommendations are also good ones. Let us know what you find.

I'll start there seems to be the cheapest solution, and I can hear air hissing under the hood but im not sure if it's good air I'm hearing, sounds like the back side of the block on passenger side against the firewell.
I will keep updated as I move along till problem is solved.

***SMWS6TA*** · 12-07-2015, 06:08 AM

There are a couple of things it could be.

If you still have it in stock configuration the EGR tube that comes from the drivers side valve cover (rear) around to the same location on the passenger valve cover then runs along the passenger side of the intake that connects to the pcv. This setup almost always causes vacuum leaks.

2nd cause back there could be the MAP sensor is crack, loose. Also since this is a engine swap into another vehicle I would look at the intake as well if you don't find an obvious leak.

When you can post up some pics. Interested in seeing a TJ with a LS1 in it.

**Hogsters** · 12-07-2015, 04:08 PM

Ok I took the throttle body off today and cleaned it up. Unscrewed the two little connectors on it and spayed some of that cleaner on them, (Im not sure what they call the connectors). I checked for cracks but couldn't find any.... it still not right. I thought Id take it to the shop but Its going to be a while before I can do that. Should I take a picture of the engine tomorrow and post it here. With the new infor for the EGR I might be able to do more, and I don't know what the IAC looks like. Is it possible that cleaning them wont work but a new one will? Looks like most of the parts mentioned here are all around $25-$35.... I can afford to do that now but cant afford the labor a shop is going to charge me.

- - - Updated - - -

Originally Posted by SMWS6TA

There are a couple of things it could be.

If you still have it in stock configuration the EGR tube that comes from the drivers side valve cover (rear) around to the same location on the passenger valve cover then runs along the passenger side of the intake that connects to the pcv. This setup almost always causes vacuum leaks.

2nd cause back there could be the MAP sensor is crack, loose. Also since this is a engine swap into another vehicle I would look at the intake as well if you don't find an obvious leak.

When you can post up some pics. Interested in seeing a TJ with a LS1 in it.

Ok Will Do, and thanks for the feedback!

**cammed goat** · 12-07-2015, 07:52 PM

The IAC is located on the driver side of the throttle body and is held in with two torx-head screws. It is chrome with a black plastic connector. Once you unplug/remove the IAC, check for build up of soot on the pintle and inspect the hole where it goes into the throttle body as the soot cakes up in there too. Use throttle body cleaner and a rag for both and get back to us with what you find.

**Hogsters** · 12-08-2015, 03:21 PM

Boy was I wrong about the part prices. So I replaced the MAF with no results. I've ordered the IAC and will put the results up on Friday after I receive it.

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**35th-ANV-SS** · 12-08-2015, 03:27 PM

Nice looking jeep. I would try to diagnose it more before just wasting money by throwing parts at it. That will get expensive quickly and might not result in any improvement.

**Hogsters** · 12-10-2015, 05:32 PM

Well I at a loss....my IAC part came in today, I installed with no results....actually rpms were higher. Don't know what next, but I cant keep replacing parts at their prices.

**RONS98TA** · 12-11-2015, 04:13 AM

You can take some carburetor cleaner and spray it along all your vacuum lines while the engine is running. If you spray across a leaking/cracked hose, the idle will go down. That is the cheapest and easiest way to eliminate all the vacuum hoses for leaks. Trying to fix a problem by just buying stuff, is NOT the way to find your problem. When spraying said carb cleaner, be careful not to spray to much cleaner on some of the plastic connectors. So try to avoid spraying any plastic. Just keep a rag handy and wipe it off asap. Some plastic is extremely sensitive to the carb spray and will get eaten up by it. The plastic will look like you took a lighter to it.

**Hogsters** · 12-11-2015, 02:33 PM

Ok so one last go around today and I find a little valve thingy that looks like it should have a hose connected to it but I cant find one, so I think maybe just maybe it suppose to have a rubber cap on it. (It's located on the back of the block in between firewall) So I find a rubber cap put it on and Wala! it runs great again!!! thanks to all those that chimed in, I 61 years old and still learning

What is the name of that thing anyways, what is it called and do?

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***SMWS6TA*** · 12-11-2015, 07:41 PM

yeah you need that. It's suppose to go to your brake booster or at least on a f body.

Not sure how yours is setup.

**RONS98TA** · 12-12-2015, 04:38 AM

That is your map sensor. (Manifold Absolute Pressure) The part # is on it. (16212460) On an F-body LS1, is it located on the back of the intake manifold, next to the firewall as you have explained. It should be plugged into the manifold with the tab that you have capped off with the rubber plug. It is possible that whoever did your install just bypassed that, or it came lose from your intake. I know on an f-body, that lil booger is a PITA to get to. If it is unplugged though, as you have found out, it will cause a major vacuum leak and make the engine run like crap. Awesome looking build by the way. That looks bada$$.

**RONS98TA** · 12-12-2015, 04:56 AM

Correction. I now realize you probably referring to the black plastic stem that the MAP sensor is connected to. Yes, as SMWS6TA stated, that stem usually has a rubber hose that goes over to the brake booster, to provide power brake assist. If you look at your brake master cylinder, there is a round black case that should have a hose coming off of it, that is IF you have power brakes.

**Hogsters** · 12-12-2015, 11:45 AM

So much knowledge here! I'm assuming since I have hydraulic steering and a brake booster as well, is the reason I'm not using the vacuum hose, and that's why it's capped off. Thanks for all the help with trouble shooting and detailed explanations. Thank You!!