Engine Please Help Me Diagnose Issues With My 5.0...

First disclaimer: Yes, this thread is for my Jeep CJ-7 with a Ford Mustang GT 5.0 engine under the hood.
Second disclaimer: I'm a Chevy guy. This Ford stuff frequently has me pulling my hair out.

Third disclaimer: This thread is likely going to get long. I have a lot of questions that I haven't been able to find answers to.


OK. Let's get started...

Problem #1

To start with, I am unsure of the exact year of the engine. I was told that it was an earlier block ('85-ish) that had been upgraded with a later EFI system ('89-ish). Personally, I think the guy who I bought the Jeep from was on crack. I do know that his brother built it, but that is about the only thing he told me that I believe. If my research has been accurate, the EFI components appear to be from an '86 or '87 car as there is no mass air flow sensor. Also, I find it very difficult to believe that someone would go through the trouble of transplanting a Mustang GT engine into a Jeep only to come back a few years later and swap in an entire EFI system complete with the computer and wiring harnesses. I suspect that the entire engine (complete with EFI, computer, and all wiring harnesses) were pulled from a wrecked '86 or '87 Mustang GT, but how can I find out for sure?


Problem #2 (hopefully not really a problem...)

The catalytic converters and most of the emissions components are long gone. It was like that when I bought the Jeep back in 2010. I would have personally kept the stuff if I had done this conversion myself. Regardless, I'm not looking to reinstall any of the stuff at this time since I rarely drive the Jeep. Here are the codes that have always been present. They have never concerned me since I understand what they are and why they are there.

KOEO: 81, 85, 82, 84, 10, 11
KOER: 94, 44, 33

Interestingly enough, the EGR valve is still mounted on the intake. It has no vacuum to it (accounting for one of the codes), but I've never known if I should keep the electrical connector plugged in or not. I'm not sure what effects either option has on the computer, so for now it has been plugged in.


Problem #3

This thing loves to kill its distributor. I estimate that in the past four years I have rebuilt the distributor six or seven times. Since the Jeep is not a daily driver, that represents a lot of work for very few miles of enjoyment. I will expand more on the specifics of this at a later time.


Problem #4

What's up with these engines and the whole surging idle thing? I'm not sure that I have ever been able to get this engine to idle properly. Recently I've gone to work on this problem again, and this time around I'm determined to make it perfect. So far I have done the following:

base idle -- set to 600 RPMs
timing -- set to 10 degrees
IAB/IAC -- fairly new and very clean
TPS -- fairly new, passed all voltage tests, adjusted to .97 at idle
vacuum leaks -- all new hoses and/or plugs installed, new PCV grommet installed
10 pin connectors -- clean with dielectric grease
fuel filter -- brand new
fuel pressure -- 40-41 psi (KOEO), 35 psi (idle), 42 psi (idle with regulator disconnected)
EGR -- clean (but see notes under problem #2)
grounds -- primary grounds checked and cleaned

edit- spark plugs have only 1000 miles on them and the spark plug wires are brand new.


Problem #5

I have found that the engine seems to run well when it is cold. The idle is smooth, and I can stab the throttle with no hesitations or stumbles. I assume this is when the computer is in an "open loop". As the engine gets hot, the idle starts getting rougher with some surging, and any stabs at the throttle result in heavy hesitations and stumbles.

There have never been any O2 sensor codes present, but as part of my process of elimination I disconnected the O2 sensor harness to see what would happen. Much to my surprise the engine idled perfectly with great throttle response when it was both cold and hot. Unfortunately I am not sure what this means. By disconnecting the O2 sensor harness, did I force the computer to remain in an "open loop" where it is ignoring all inputs (including the ECT sensor, for example), or was the computer only ignoring input from the O2 sensors (which would seem to indicate that they are the problem)? I need to know what effect unplugging the O2 sensor harness had on the computer.


Problem #6

Last night I had the engine idling for quite a while. Quite unexpectedly it died and then wouldn't restart for almost half an hour. My gut tells me this is another ignition control module on its way out. (See problem #3.)


Problem #7

On rare occasions I get code 67 (neutral pressure switch or clutch switch circuit failure) when doing a KOEO test. This of course prevents me from doing a KOER test as well. The code usually goes away fairly quickly, but I read some discussions on how code 67 can cause a surging idle. The transmission in the Jeep is a manual with "granny low" from an older Ford F-150 or F-250. There don't appear to be any wires coming out of it, so I'm not sure what occasionally causes code 67 or how to troubleshoot it.

More to come...

jrichker said:

Go to it.. That's what we are here for.

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Thank you. I certainly appreciate any help you and the others here can provide.

I realize that I posted a ton of information this morning, guys. Please don't let that overwhelm you or discourage you from replying in this thread. I assure you that I'm mechanically inclined when it comes to working on vehicles and very logical in the way I think. If you have questions or need more information before giving an answer just ask. Feel free to respond to any or all of the problems I listed if you have something to contribute.

If it is generally preferred to keep the discussion to one problem at a time, I'd personally like to start with problem #5. The fact that the engine runs worse when it is hot has me stumped. Also, before I go spend $90 on some new O2 sensors I need to know why the engine ran better when I disconnected the O2 sensor harness at the firewall.

madspeed said:

have you done the base idle reset and checked out the surging idle checklist?

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Yes. The surging idle checklist was a huge help. Take a look at my list of what's been done under problem #4. I actually did find some vacuum leaks, so fixing those was somewhat encouraging.

By the way, I just noticed that we can't edit posts after 15 minutes. That will make this thread even more confusing ... LOL! I wanted to add under problem #4 that the spark plugs have only 1000 miles on them and the spark plug wires are brand new.

madspeed said:

#4 edited

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Thank you.

Treachery said:

Here's a couple of things to look at.

problem #2 (EGR)- with the EGR is still present- OK for the electrical is plugged in. I would leave the EGR plumbing, if the EGR is still present.

problem #4- Base idle should be set for 850 rpm. Adjustable by the TPS voltage.
TPS- should be between 0.90 -0.99 - 0.98 is the sweet spot. I'm set for 0.99.
Fuel pressure- should be 39lbs - 40lbs. Vacuum off.
Spark plug- assuming you have the correct type- gap .35 for starters.

A dynotune should be done. A tune should fix some things and will also reveal any potential weirdness as well.

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Thank you for replying.

Problem #2: In that case I will keep the EGR electrical connector plugged in. There is nothing remaining for the vacuum to be plumbed to, so that will have to remain inoperable.

Problem #4: Can you please point me to where there is documentation showing that base idle should be set at 850 RPMs? Everything I found indicated to set it at 600 RPMs with the IAB/IAC disconnected (some sources also said to pull the SPOUT, which I did do). I would personally love a higher idle (it makes rock crawling with a manual transmission much easier), but I kept getting code 12 whenever I had it set much higher. Also, am I right in thinking that my fuel pressure is OK even if it is slightly higher than what you are indicating it should be?

Treachery said:

... <snip> ... After following the Idle Surge Checklist, did you reset the computer? ie- disconnecting that battery. wait about 20 minutes. then reconnect and drive. The computer will then relearn its idle strategy.

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Yes, the computer has been reset numerous times.

Treachery said:

Stock fuel pressure is 39 lbs vaccum off. When you reconnect the vacuum, the fuel pressure should drop a little bit. Your fuel pressure appears to be OK. For troubleshooting purposes and personal voodoo, I would adjust it accordingly.

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I hope the fuel pressure is OK. I have no way to adjust it.

Treachery said:

You might also want to inspect and clean the Idle Bypass Valve. The cylindrical thing that hangs off of the throttle body. Spray the inside with carburetor cleaner, let dry and reinstall with new gasket. Idle will be a rough at first as the computer relearns but will eventually smooth out.

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Please see what I have already done under problem #4. The IAB/IAC is relatively new and very clean. I even removed it again to confirm this.

Treachery said:

It is possible to set the RPM at 850 rpm via the Idle Screw on the throttle body but it will throw off the TPS voltage and you will have to go through again the entire Idle Surge procedure. It may open up another can of worms so don't do it unless you feel adventureous. It sounds like your Jeep is a recreational vehicle- ?

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Aside from the missing emissions equipment, the engine is all stock. I'd like to get the engine running correctly with the factory settings before I venture into making any tweaks on it. It didn't seem to like it the last time I had played around with a higher idle setting.

Yes, my Jeep is strictly a recreational vehicle.

Treachery said:

On the '86 and earlier 5.0 engines... They came out of the factory with a Speed Density setup. Do a search for the explanation. There is a conversion kit from Speed Density to Mass Air which requires a change of the EFI system, computer included. This was what the previous guy most likely did. The change from Speed Density to Mass Air was done mainly to reduce smog and stuff like that.

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I don't understand what you getting at here. My engine is a speed density setup. There is no mass air flow sensor on it.

jrichker said:

See the surging idle checklist below for a starting place...

... <snip> ...

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Done, done, and done! Your surging idle checklist is how I first found this forum back in 2010, and I have been frequently referring to it ever since. The things I have already tested and fixed (listed under problem #4) are a direct result of the information contained in the surging idle checklist.

There are a few more parts I need to test, but I'm stuck on the O2 sensors right now. Why did the engine run better with them unplugged?