Figure this out and you are a Jeep GOD; hesitation in traffic but runs fine on hwy
Iíve been in Afghanistan for a few months and when youíre in a place like this, you have a lot of time to think about stuff. And Iíve been thinking about my CJ7 a lot; where Iím going to drive it, what parts Iím going to buy, and all that. I canít wait to take my son for a drive out to the beach or around the base on the trails when I get home.
I have a question about the jeep thatís been on my mind and Iíd like to know where to start when I get home. Before I left, I was experiencing a strange problem that no one seems to have an answer for. Here goes.
Jeep starts and drives fine. Drives great for long distances, doesnít overheat, doesnít stutter or anything on acceleration.
However, when I drive it for 10-15 minutes, it warms up, and I get in stop and go traffic (usually on my way home from work) it begins to run very rough. Acceleration stutters and it feels like everything is just off, like itís trying to limp home. When I get it home and let it set for a few hours, it starts and drives just fine again.
The í84 CJ 7 is all stock, 4.2L 6-Cyl, Carter BBD Carb.
I have done these things so far:
1) It does not overheat, but: Changed thermostat, flushed cooling system
2) New fuel filter Ė checked carb jets (they are clean)
3) New Electronic Control Module, distributor cap/rotor, plugs, wires
4) Swapped out almost all the Vacuum tubing and rubber fuel line
5) Nutter Bypass because I thought it could be a computer problem
Thanks for taking the time to read this. Any thoughts?
Vapor lock? Maybe a fuel line run kind of close to some of the exhaust? At highway speed, ample air flow may be keeping the line cool enough but in traffic, heat builds up and the fuel begins to boil in the line causing symptoms similar to what you describe? I'd at least check fuel line routing to eliminate vapor lock possibility.
'80 CJ5 w/AMC360, T176, D44 w/Detroit 4.56, RE 4" YJ lift and 35s..
Welcome to the forum, and thank you for your service!
I think your problem is with the EGR. They get dirty and stick or just leak.
There is probably an EGR switch or a CTO switch involved in switching vacuum to the EGR once the engine warms up. I'll see If I can dig up a scematic of how it works, but for now I'd test the EGR by putting a vacuum to it with the engine running. See if the idle changes.
Edit: See the EGR CTO in the upper right? That senses coolant temp, and switches on/off vacuum to the egr. The switch seems to be working, but the egr opens and lets exhaust gasses into the intake. Try cleaning the EGR first. Could also be a source of a vacuum leak.
'85 CJ7, BDS 4" lift, 1" Body lift, 33x12.5, Shrockworks Sliders, 304 V8 with RV cam., T-176, D300, Dana 30, AMC 20. 1986 CJ10-A SD-33 Diesel/727/np208 1971 800B with 345/T-19
06' TJ Rubicon, 4" R.C. springs, BFG/AT 35s M.C. 6" fenders, rockers and surrounds, Currie front & rear adj. tracbars, tattons DC rear shaft, adj. upper - lower CA's. Bilstein 5100's YJ Buggy Build Current project. Stroker/FI ?
1990 MJ Comanche 4.0L AX15/np231
Last edited by BioTex; 08-04-2010 at 07:54 AM..
Reason: Added diagram.
Many thanks for your service! My cousin is headed back there in just a few short weeks. Hence, my avatar.
I like Biotex's EGR possibility but I'd like to throw something else in the pot to ponder. I had a similar situation when I first got my Jeep. It would start up pretty well but within a few minutes it was running very rough. Let it sit for a few minutes and she was good to go.
Turns out the intake manifold gasket was bad. I guess as things started warming up, different metals and thicknesses would expand at different rates and the gasket would have a major vacuum leak. Let it heat soak for a bit and things would get to their final, expanded size and the leak would disappear.
__________________ Live in a way that those who know you but don't know God will come to know God because they know you.
Last edited by swatson454; 08-04-2010 at 10:30 AM..
Vapor lock? Maybe a fuel line run kind of close to some of the exhaust?
Sounds reasonble. Make sure your fuel filter is installed correctly. The filter should sit almost parallel to the ground and the 'vent' line should be on top. Make sure the vent hose is routed properly back to the gas tank and the line is not blocked.
"I give you a republic, if you can keep it." - Benjamin Franklin
Really sounds like a fuel issue/heat soak problem in a way. Stop and go driving elevates temps under the hood and fuel is not being pumped as fast through the lines. Like mentioned above, check your fuel filter and its oreintation and the fuel lines. Every sticking EGR valve that I have had caused a low/rough idle but always accelerated fine etc, but it is worth taking a look at.
Hey there, new member here. I've been using the heck out of the search function for my own issues, maybe I can help for a change.
My 85 CJ also has the Carter BBD. A few weeks ago, it was doing similar stuff. I had a new air filter element, but hadn't replaced the pcv filter that goes in the slot inside the filter housing. I pulled the PCV membrane and it was filthy. Put a clean one in and no more hesitation or shuddering.