'85 CJ7 Curb Idle Mystery

'85 CJ7 Curb Idle Mystery


Southlake, TX
1985 CJ7 [Inline 6, Weber 38/38 DGES Carb]
My 1985 CJ7 went from running fine (over several years with the same settings/config) to not wanting to idle (pre-problem for me 800 RPM on a good day, 900 nominally), overnight. There were no changes, and no noticeable problems the last time it was driven. The next time I tried to start it, it no longer wanted to idle. The following is a chronological order of events and attempts to troubleshoot the problem so far:

Engine starts up (as it always has, i.e., no change in ability to start) and runs fine through the fast idle. However, once the electric choke heats up and opens the butterfly, the engine starts to stumble and eventually die.

1. Did a quick visual inspection of the engine bay for anything obvious. Notice that the black potting material on the back of the ignition module has melted (not sure when) and run down the inside fender. Ah ha, that must be it... but no. Replaced ignition module - no change in engine idle performance.

2. Replaced ignition core - no change in engine idle performance.

3. Well, it must be the carb then, right. So I pull out my weber 38 DGES printed spec and begin to re-check/tune the air/fuel mixture screws and idle screw. After changing things well off what they have been in the past, I was able to get the jeep to maintain an idle but way too high, like near 1500 rpm. No bueno. Carb adjustment - no change in engine idle performance.

4. Decide to pull the carb and do a complete inspect and clean of every jet/venturi/orifice/etc. Cleaned carb with carb cleaner and compressor/air. There was minimal residue, but nothing that lead me to believe anything was clogged. Reassembled carb, checked all linkages for binding and reset. Carb clean, no change in engine idle performance. I can clearly see two full streams of gas enter the carb body with a pedel press. Butterfly opens as it should as the electric choke heats up. Throttle linkage not binding. (Carb is a Weber 38/38 DGES I purchased new from Weber about 2-3 years ago - been running fine since, I've successfully tweaked the settings over that time depending on seasonal temp changes, etc without any previous issues)

5. Well, maybe I've got bad gas. So, I drained all the gas and replaced with fresh gas - no change in engine idle performance.

6. Replaced fuel filter - no change in engine idle performance.

7. Ok, maybe the gas isn't getting to the carb. Pull the gas fill line at the carb and crank the engine. Good steady stream of gas, approximately 6-7 oz in 10 seconds. (mech fuel pump 2 years old) I don't have a fuel pressure regulator (FPR) on my Weber, but this distribution of gas seems consistent with previous working configurations (for me). I attempted to install a FPR long ago per Weber's advice but seemed to run better (for me) without it installed. May have been then cheaper FPR I purchased - don't know. Either way, I haven't had one in a long time and don't currently have a FPR installed.

8. New spark plugs, why not - no change in engine idle performance.

9. Grasping for straws. The catalytic converter seems like it may be the original. Thinking it must be plugged up and causing if not the main problem, a secondary problem. Replaced the cat with a new Magnaflow catalytic converter. Additionally, during the replace I noticed that there was a pretty severe exhaust leak at the (bolt together) flange between exhaust from engine and entry to original cat pipe. I'm mentioning this here mostly to be thorough... I took the opportunity to add a new flange gasket but there was still some leakage, so I sealed around the entire flange with exhaust sealer so now everything is completely (for the most part) sealed through cat/muffler/exit. Since I'm chasing ghosts at this point, I'm starting to worry that sealing that up may be causing some additional amount of "back pressure", if that's possible, that the engine cannot handle now - and paranoidly thinking that the previous owner intentionally left the flange vented b/c of this - probably not, but I've been at this for a while now and starting to lose my mind). Either way, new cat install - no change for the better wrt engine idle. Substantial decrease in engine smell in cab though.

10. The infamous vacuum lines. Performed laymen’s check of vacuum lines - never had a problem previously, they all seem to still be connected, no deteriorated hoses, sprayed carb cleaner around all connections, pinched off each hose - really no change to engine idle performance while jeep is running. In an associated move (and I'm not sure the EGR is even in play at this point in this discussion), but decided to pull the EGR. I was not able to move the diaphragm by hand. Sprayed copious amounts of carb cleaner in the small holes around the EGR circumference. Started working the diaphragm by hand, more carb cleaner... eventually got good movement from the EGR diaphragm. Reinstalled - no change in engine idle performance. Assuming vacuum lines are not the primary issue with my idle.

11. To Nutter or not to Nutter. All my friends with CJ's have done it, I always assumed my jeep's previous owner had it done but never actually checked the wiring to verify it had been done. Checked wiring and appears that the Nutter bypass had NOT previously been done. Well, maybe that will help things. Maybe the old PC crapped out and was hosing something up. Didn't take long to pull the diagrams off the web, performed the bypass - no change in engine idle performance. (leaving config in nutter bypass moving forward however)

12. Running out of obvious things to check so I pull out my timing light. Before I even hook it up, I know the timing isn't per spec but I also know that the timing it has, has kept my jeep running solid for the past 5 or so years. Since I just performed the Nutter, I'm thinking I need to get this thing to 8* BTDC from what I've read. Well, the jeep is not having any of it. It just will not stay running at 8* BTDC (distributer vacuum pulled and plugged / taking timing light measurements @ 1600 RPM). Played around with the timing for a while trying different values, but ultimately none of it fixed my curb idle problem. I set the idle back close to where it was before all this started (~ 12 BTDC) so that I could at least maintain a curb idle to continue to troubleshoot. Tried a window of timing setting - no positive change in engine idle performance. Moving on for now.

13. Day 3, frustrated so I play with the timing and carb air/fuel screws (x2), high idle screw, and idle screw some more before trying something else. Eventually get some kluged configuration such that the jeep will maintain idle around 1200 parked in the garage. Decide to drive it around a bit and see what kind of results I get. Drove fine, except every time I started breaking at a stop sign the engine would stumble and 90% of the time die if I didn't apply the 'ol "gas/clutch/break - gas/clutch/break" maneuver. After I was stopped however, the idle would maintain itself. This was very repeatable at each stop. Returned home, defeated.

14. Clean once, clean twice - I'm confident that I did a thorough job of cleaning the carb, but I neglected to take float measurements and compare against the Weber spec. Maybe the float is off since as I'm breaking it starts to die. I'm thinking in my head the gas in the bowl is tossed around as I'm breaking and the float is maybe incorrectly set. (not sure how that would happen by itself overnight, but whatever) Off comes the carb again. Complete clean of carb again. Can't remember what the float measurements were other than that they were not what the Weber spec called for, so I reset (my Weber has the plastic floats) per the Weber spec - measured to within 0.5 mm on both measurements. Bolt everything back together, cross fingers - no change in engine idle performance.

15. OMG! I forgot to replace the PCV valve. I bet that's it. $2.15 later I have a new PCV valve installed and the same problem as before.

16. Compression Test Results - I decided to perform a dry and wet compression test of the cylinders in an attempt to further diagnose my issue. The results lead me to believe that engine wear is not the cause for my jeep not idling. Readings listed are averages taken over 3 samples of each cylinder both dry and wet, engine hot @ WOT with the ignition coil disconnected and all plugs pulled. Wet performed with 1 tbsp oil in each plug.

Cylinder 1: Dry 152, Wet 180
Cylinder 2: Dry 153, Wet 182
Cylinder 3: Dry 154, Wet 185
Cylinder 4: Dry 152, Wet 170
Cylinder 5: Dry 157, Wet 179
Cylinder 6: Dry 150, Wet 181

.... So, I'm not really sure where to go next. I need some guidance/advice on what could be driving this idle issue and what to explore.
Wow man... You've been busy...

I have only guesses for you, and they're not even informed guesses, just ideas...

have you checked every fuse and relay? They do have them that year right?

What kind of distributor? I assume mag pickup? Check cap and rotor and all that,
That has to be the most beautifully written tech question Ive ever seen. :notworthy:

I unfortunately have no answer for you. I am however paging Dr. BusaDave9 to this thread for a consultation. Please stand by. :)
WOW, you've tried everything. I mean everything. It's almost time to double-check what you've already checked. When I was reading your post I was thinking it's a vacuum line. Then I got to the part where you thoroughly checked vacuum lines. You could try removing vacuum lines and plugging them. You don't need any of that at idle. No vacuum advance won't matter if you are checking idle speed.

I really don't know what your problem is but if you will bear with me I would like to cover what I don't think it is. I just want to think out loud to help narrow it down. I don't think it's electrical. I don't think it fuel delivery or spark. These issues would be more pronounced when you are accelerating or under power not at idle. I think it's more likely to be air. More specifically the air/fuel mixture. That's why I was thinking vacuum. Maybe carburettor but you've covered that very well. I really don't think it's the float level. Once again that would show up under heavy load.

One thing that stands out is your timing. You say it won't run at 8 degrees. It's odd that you have to go to 12 degrees.So now I think your timing chain and sprockets are worn. Maybe your timing chain jumped a tooth. That would mean your timing isn't really where your timing light says it's at. Your valves would also be off.
Didn't have much time tonight, but did manage to replace cap and rotor just to close out the ignition chain.

I guess I'll revisit the vacuum lines. Is there a good diagram available for a 258/weber/nutter config. I still have charcoal canister and 4 port CTO.

Are there any sensors that could have gone bad and fouled something up, or is that all out of the loop with the nutter bypass.

I'll also try to investigate the timing chain jump, but that may have to wait till the work week is out when I have a decent block of time.
Ok, well I thought I'd draw this thread to a close since I came to closure over the weekend. Maybe it will help someone else with the same symptoms.

Proceeded to replace the distributor core since I had replaced everything else up to it. Still no change. So I bit the bullet and starting pulling parts to inspect the timing chain. Not knowing exactly what to look for, I found that the timing marks were lined up. However, there was well over 1" of slop in the chain (didn't measure). Turns out the whole timing set is only $49.99 at Oreilly's so I popped a new set in and bolted everything back together. Not thinking this would fix my problem I press the pedal once and turn the key over. Jeep fired right up, set timing to 8* BTDC and adjusted the carb. Jeep now idles at 600 rpm without issue. I just kinda stood there with my jaw on the floor, asking myself why I didn't do that 4 years ago. It seems to have fixed every issue (high idle, rough idle, hard cold start) I've ever had. Maybe it was a combination of new timing set and all the other new parts... don't know, but happy to have it running again... and better than it ever has since I've owned it.
Glad to hear you figured it out. Now you have everything upgraded around it. Doubt that money was wasted , Im sure it contributes to the smooth runner you have now. :cool: How many miles were on it? May have to check on mine as well.
I once had a bizarre miss and couldnt find the culprit...UNTIL I had it running in a very dark garage, and noticed a very faint spark coming from a plug wire arcing off the manifold....If I hadnt been in the pitch black garage, I wouldnt have noticed it.

It would be nice to know what the problem really was... glad it is solved though.
I had it running in a very dark garage, and noticed a very faint spark coming from a plug wire arcing off the manifold....If I hadnt been in the pitch black garage, I wouldnt have noticed it.
everyone should watch their engine run in the dark. It's an easy way to check spark plug wires.
If you spray them with soapy water the arc will be more pronounced...
Good to know. Thanks Phyrph0enix. There aren't many ways to test the insulation of spark plug wires but that's a good idea I have never heard of before.

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