MayDay!! - Hard Accel Killed Idle

[AnotherOldJeepGuy]

I think I kinda get it.  The throttle "opens a big hole" on purpose to the outside air and that would cause the vacuum to drop.  I didn't think about that.

 

I tried plugging off the line to the brake booster back on April 10 (hard to believe this problem has now been in place for almost 2 months) and nothing changed, but some improvements have been made since then and maybe it's time to repeat that test.

[AnotherOldJeepGuy]

23 hours ago, Ωhm said:

88 FSM Computer Relearn Procedures.pdf

Tried the reset procedure, no change 

[AnotherOldJeepGuy]

Took Jeep in to a second shop for more diagnostic assistance, this is a shop I used before for a muffler, and the primary mechanic has been there for 30 years, so he actually has seen this engine.  I let him know all the stuff that has been done, and also that at least one goal was assistance looking for vacuum leaks.

 

He pointed out two things before smoke testing.  One was the injector harness connectors looked corroded and not very tight.  (This could relate to the REM readings about injectors).  He actually was able to pull them off without loosening the retainers, I don't know that you should be able to do that, but he did with no trouble. He recommended they be replaced.  Rock Auto sells them for about $8 each.  The other was that he found one of my new plug wires arcing at the plug end boot to a vacuum hose (the one that goes to the heater control valve) that decided to rest next to it.  So he suggested that wire also be replaced.  My wires are new, I suspect I poked a hole trying to remove them when I pulled the plugs out a week ago, but still I guess I'll replace that one.  I find it hard to believe either of these failures are a result of the maneuver incident, but since I have not given up that my truck will survive this yet, so might as well fix what needs fixed whether it has anything to do with the bigger problem or not, particularly if they are fairly easy and inexpensive.

 

Then on to the smoke test.  He introduced the smoke at the port just in front of the throttle body, the one that normally connects to the hardpipe going to the back of the valve cover.  We let the smoke build up for a while and the only place any smoke came out was the airbox.  My assumption is that location is OK, I am assuming the airbox is not expected to be completely air tight.  The mechanic didn't seem to be concerned about the smoke there.  We saw zero smoke anywhere else.  He raised the truck up so we could also look from the bottom, and still no smoke.  So unless the smoke entry point used is not valid, there does not appear to be any leak in either manifold or the gasket.

 

I asked about the potential of timing being knocked off by my truck-killing maneuver.  The plan is for me to go forward with the items above first, then once they are complete and if they don't fix the issue, I'll take it back and he will break out the timing light and check the timing.  My guess is, I will be taking it back for that timing check, but if the items identified here do fix it, I'll take the win.

 

@Gojira94, I'd like to hang on to the REM until I put on the new injector harness connectors if you don't need it back sooner.  Would be nice to see if the INF errors disappear.

[Gojira94]

17 minutes ago, AnotherOldJeepGuy said:

I'd like to hang on to the REM until I put on the new injector harness connectors

 

Absolutely, hang on to it.

[Eagle_SX4]

21 minutes ago, AnotherOldJeepGuy said:

I asked about the potential of timing being knocked off by my truck-killing maneuver.  The plan is for me to go forward with the items above first, then once they are complete and if they don't fix the issue, I'll take it back and he will break out the timing light and check the timing.  My guess is, I will be taking it back for that timing check, but if the items identified here do fix it, I'll take the win.

As I understand it the timing is not adjustable on the 4.0. It is either good or bad. The distributor needs to be indexed properly and the computer will take care of the rest. It is easy enough to check if you distributor is indexed properly.

 

 

 

[AnotherOldJeepGuy]

27 minutes ago, Eagle_SX4 said:

It is easy enough to check if you distributor is indexed properly

Yeah, I have read through those instructions about 10 times, but I get lost at about step 7  (well assuming I could get past Tip 12, and not very confident on that either)

 

27 minutes ago, Eagle_SX4 said:

timing is not adjustable on the 4.0. It is either good or bad

Given this, is there any logic to the thought that what I did could have changed this?

[Eagle_SX4]

1 hour ago, AnotherOldJeepGuy said:

Given this, is there any logic to the thought that what I did could have changed this?

Timing is not something that changes suddenly unless there was a failure of the distributor or cam shaft. I remember you saying you wiggled the distributor rotor with the cap off and it didn't move much so I don't think you have an issue there.

[AnotherOldJeepGuy]

Great, and that makes sense.  I'm going to assume for now at least this alignment has not changed since before whatever it was that happened, happened.

[AnotherOldJeepGuy]

Here's a new thought for comment by the experts.  A friend at the gym when I told him my story, including what kicked it off and all the stuff done to date, said "are you sure you cat converter isn't damaged?".    I asked what it might be about this that could cause issues like mine, he said if a piece broke inside by the jolt it could limit airflow.  This makes as much sense to me with my degree of expertise as anything else that has been looked as so far.  And it is known that after my maneuver my muffler was broken at the input pipe, it would not be a stretch to think the jolt that did that might have damaged the 36 year old converter.

 

What's the thoughts?  What would be the best way to check this?

 

If it was damaged, what would happen if I just bypassed it, I have no inspection requirement.

[llhat]

been running a gutted housing for i can't remember how many years....   no inspection requirements for me either 'now' but during my life with it, would always pass a visual,  did not have Gas testing here.

[AnotherOldJeepGuy]

32 minutes ago, llhat said:

been running a gutted housing for i can't remember how many years....   no inspection requirements for me either 'now' but during my life with it, would always pass a visual,  did not have Gas testing here.

Ah, so it sounds like if I did bypass it, nothing bad would happen (it would not make it run differently)

 

Remaining questions still out there....

[89 MJ]

I’d just cut it out and replace it with regular exhaust pipe. You’re really not going to get any negative effects. You can get aftermarket cats if you decide you want one in the future. 

[87MJTIM]

When my cat crumbled on the inside my power and acceleration really suffered. It would idle just fine, it just couldn’t keep up with its own shadow. 

[AnotherOldJeepGuy]

11 hours ago, 89 MJ said:

I’d just cut it out and replace it with regular exhaust pipe. You’re really not going to get any negative effects

 

11 hours ago, 87MJTIM said:

When my cat crumbled on the inside my power and acceleration really suffered. It would idle just fine, it just couldn’t keep up with its own shadow

I'll put this on the near term todo list then.

[Ωhm]

On 6/11/2026 at 4:21 PM, AnotherOldJeepGuy said:

Given this, is there any logic to the thought that what I did could have changed this?

 

Distributor could have jump a gear on the camshaft. 

[AnotherOldJeepGuy]

12 minutes ago, Ωhm said:

 

Distributor could have jump a gear on the camshaft. 

That was one thought I had, well in my terminology it was could I have gotten the timing off.  I know there is the dist index post, that I have read many times now and don't believe I understand it good enough to do it.  So if one had a timing light, could one tell if this was off with that?

[Ωhm]

43 minutes ago, AnotherOldJeepGuy said:

So if one had a timing light, could one tell if this was off with that?

No, if your timing was off, it would be the fault of the ECU or ECU inputs.

 

 

Timing is controlled by the ECU. Indexing is where the tip of the rotor is in relation to the distributor tower terminal. Crossfire, weak spark are some of the symptoms. Try and determine if your setup (index) is like that as shown in the photos. No need to cut tabs or remove distributor. Just check indexing. 

[AnotherOldJeepGuy]

1 hour ago, Ωhm said:

Just check indexing.

Is this possible without stepping through tips 12 and 13 which I still don't believe I understand well enough to do.

[Ωhm]

Is this the tip 12 you're talking about?

 

Tip 12

 

[AnotherOldJeepGuy]

Yep, that's the one!

[Ωhm]

That timing mark on the crank (0°) can show up on either TDC for Cyl#1 or Cyl#6, depends on where the camshaft (valvetrain timing) is. Need to determine which one. Tip 12 tells how to do that.

[AnotherOldJeepGuy]

Unfortunately that 12 is the 12 of 12&13 I don't believe I understand well enough to do. .  I'll go do some youtube searching and maybe find some examples.  Pretty sure I can't get my finger or thumb down in the plug hole, definitely not my thumb, while at the same time turn the engine at the balancer bolt with the other hand!

[Ωhm]

Yea. Plugging the hole and rotating the engine needs two people. Let see if I can make this even more confusing. Instead of Cyl#1 plug, remove Cyl#4's plug (easier to get to). Then the next time the timing mark shows up, you're on TDC compression stroke Cyl#1.

[AnotherOldJeepGuy]

If the timing mark is the mark on the balancer, wouldn't the "next time the timing mark shows up", which I am only guessing mean shows up lined up with the line(s) on the block, be visible just the same with or without the plug in?  Not sure what difference the plug being in or out is on if the timing marks show up, so yeah, more confusing bar reached .  I guess the day all this timing stuff was taught during my Advanced Shade Tree Mechanic class I must have been out.  Either way it has hit mid 90s out in my driveway so I am done for the day out there, I'm too old to work out in the sun!  I'll start again in the morning.

[Ωhm]

Plug removed, thumb over the hole, only tells you when that cylinder is on the compression stoke. Time to fire (ECU) the plug (distributor) for that cylinder.