Hey guys, I am having some serious issues that I can't find the source of. Have a 1990 Cherokee Sport with the 4.0 (4WD), 210,000 miles. First of all my idle air control has been opening up all the way when I start up on a cold engine. I had replaced the valve about a year ago when I had the same issue which turned out to be a vacuum leak to my EGR valve. Tried spraying come carb cleaner into the idle air control valve, which isn't the issue, because as soon as the engine starts to warm up the valve closes. I had put a piece of duct tape covering 90% of the opening to the idle air control valve and that worked for a little while. Problem started happening again, where it was idling at 1500-2000 RPMS on a cold engine...sometimes it would come down after driving for a while, and sometimes it would not. Also, my gas mileage which I started keeping an eye on dropped from its normal 20 MPG, down to 10-12 MPG (I can smell the rich mixture, although it isn't smoking). I started inspecting the vacuum system for any leaks by unplugging the brake boost line and plugging it...that seemed to drop the RPMS back down, so I thought I nailed it. Was gonna pull it into my garage this morning to remove the master cylinder and brake boost, but before I moved it, I removed the duct tape over the idle air control because I figured it would now work properly. Went to start it and it ran for a few seconds and it flooded out. Repeated starting it and flooded out a few times. Then I plugged the idle air control valve back up again and it started. Got it into the garage and noted the RPM at 1000 with the idle air control plugged and the brake boost line plugged. Put the brake boost line back to the the brake booster and the RPM was still at 1000. So the problem isn't the brake booster. Something is causing the ECU to both open up the idle air control too much, and enrich the mixture. I also ruled out any other vacuum leaks because I removed the vacuum lines one-by-one (except the one for the MAP sensor) and plugged them. There was slight drop in RPM as one would expect but nothing significant. I think the problem could be one of three things: 1. Bad MAP sensor 2. Bad MAT sensor or 3. Cracked intake manifold. What do you guys think before I start tearing things up and spending lots of money?????

Thanks in advance for your help!!!

 

Wish I could nail your problem on the head but maybe I can help you troubleshoot. Of the three possibilities you mentioned, I would lean more towards a faulty MAP sensor or intake leak. The MAT sensor shouldn't alter the fuel supply so much that it causes the engine to flood. You might check www.lunghd.com for some sensor troubleshooting, and check "Vacuum Leak Diagnostics" on that page as well. It sounds like you're trying to jury rig the emissions control with duct tape and those IAC's are somewhat delicate... I would get one you're sure is working order, sometimes they come defective right out of the box. I assume you have a Renix system, and they're notorious for ground faults, so check your ground straps to be sure all of your sensors are getting quality electrical contact. Next thing I can think of is to check your injectors & make sure they are spraying the fuel well. There are so many variables to the emissions control on a Renix it helps to isolate some of the possible culprits first.

 

Thanks for the advice. Went on to that website and got some good info for checking grounds. Checked the engine ground and it was good. Checked the ground for the MAP sensor and the MAT sensor and both were good. Ran the engine and poured a small amount of water on the manifolds where they meet the block and the engine did bog down indicating that there is an air leak between the manifold and the block. That explains the idle issue, but would that also explain the rich mixture? I thought if you introduced extra air into the manifold without the TPS registering application of throttle, that it would lean the mixture, not richen it. Looks like I got some work to do. Is pulling the manifold as bad as it looks??

 

Doesn't really look all that tough, just a little time consuming. I'm getting ready to do all that when i swap-out my Header next weekend.

Here's a Tech Write-up from another forum on Header replacement, no need to go that far, but it would be a good place to start...

http://www.cherokeetalk.com/forum/f2...p-header-4171/


Good Luck!!!

 

Thanks for the help...one more thing....when I went to start it up to move it out of the garage a little while ago, it really didn't sound like it was running good at all....Lots of vibrations like it was miss-firing. I wonder if the water that got sucked into the defective manifold gasket could have either fouled my plugs or caused vapor-lock type damage to one of the cylinders. Or maybe its just running rich as crap.