MayDay!! - Hard Accel Killed Idle

[AnotherOldJeepGuy]

I made the "plug" below.  The bolt is in very tight to start then snugged further with 2 tie wraps.  I don't know that this was a 100% plug but it should be very close to it.  (I can redo it with come E6000 glue if it doesn't look like it is OK as is).  With this one the vacuum port (but before reading your post about resetting by disconnecting the battery) I saw no change.  Sluggish response to throttle, running choppy, and at least once when I got the RPMs up and let go I got a small backfire (similar to the one I posted a video of a page back).  I'll go disconnected the battery and repeat the test.

 

[AnotherOldJeepGuy]

The battery reset didn't change the outcome, still sluggish response to throttle and running very choppy.

 

My son has a thing called a soap cannon he uses to was his car.  I'm gonna see if he will bring in over and let me use it to suds up the exhaust real good and see if any bubbles show up.  Think I'll need to do that first thing in the morning when it's as cool as it gonna get since the exhaust is gonna dry out pretty fast once the motor is running, but just maybe I'll see something useful.

[AnotherOldJeepGuy]

Heading out to get the tools for checking the fuel pressure.....

[AnotherOldJeepGuy]

Got the tool from Harbor Freight and measured the pressure at the fuel rail.

FPR hose attached = 30PSI

FPR hose not attached = 37PSI

[Gojira94]

Fuel pressure is fine, not an issue. A piece of hose and a bolt is a time-honored method of plugging a vacuum port lol. I'd say leave that in place for now while troubleshooting continues. There are plenty of places for vacuum lines to crack near the vacuum tank and the check valves may not be healthy enough to hold vacuum. Once the issue is solved, you'll want to investigate and patch up any other other vacuum line breaks, and see if the vacuum tank still holds. Maybe even get cruise control back...

 

Meantime, I think you're on the right track to try to confirm a cracked exhaust manifold. And the O2 sensor may be dead or contributing to the issue if it's commanding more fuel if it's seeing what it thinks is a lean condition from cracked manifold. When you turn the key on prior to engine start, do you see the 'Check Engine' lamp light up during the lamp check? I wonder if the bulb might be burned out and would otherwise be illuminated.

 

I'm willing to loan you my REM, I don't have any immediate need for it. PM me with info and I'll mail it to you.

 

I'm still concerned about verifying spark/ spark timing. More so with the distributor indexing than anything else. The coil and module are working, at least. The coil can be tested for primary and secondary resistance easily enough with a meter while the module would require a DRB II.

 

 

[Ωhm]

2 hours ago, Gojira94 said:

I'm still concerned about verifying spark/ spark timing. More so with the distributor indexing than anything else

 

Poor spark can cause plugs to run RICH.

 

 

[Gojira94]

One more thing - pull the distributor cap if you haven't yet. Leave the wires on it, and have a look at the rotor and the cap contacts. And try to rotate the rotor by hand. It should have a really tiny bit of wiggle to it but not a lot.

[AnotherOldJeepGuy]

21 hours ago, Gojira94 said:

When you turn the key on prior to engine start, do you see the 'Check Engine' lamp light up during the lamp check?

AFAIK I don't have a Check Engine lamp, I've never seen one in over the last 36 years that I can recall (which in most cases would be a good thing ).  I looked in my manual and it doesn't identify one existing at least not in the instrument cluster.  The closest thing seems to be the Engine Maint light that apparently went on at 82,500 miles, that was way too long ago for me to remember it, but I assume it did.

 

21 hours ago, Gojira94 said:

O2 sensor may be dead or contributing

I replaced this sensor back in Feb 2025 when I had an idle issue.  The issue turned out to be very dirty TB and IAC and was completely resolved after cleaning them and putting in a new TB gasket.  Of course it could still be contributing, just not because of age.

 

16 hours ago, Gojira94 said:

One more thing - pull the distributor cap if you haven't yet. Leave the wires on it, and have a look at the rotor and the cap contacts. And try to rotate the rotor by hand. It should have a really tiny bit of wiggle to it but not a lot

I'll give that a shot!

 

Have not coordinated with my son on the soap cannon yet for the bubble test but hopefully can get that done pretty soon.

 

19 hours ago, Ωhm said:

Poor spark can cause plugs to run RICH

I'm guessing I would need another tool I don't have to test this.... correct?

 

Thanks for all the assistance so far!

[Ωhm]

7 minutes ago, AnotherOldJeepGuy said:

I'm guessing I would need another tool I don't have to test this.... correct?

 

Secondary Ignition (High Tension) consists of coil wire, spark plug wires, distributor cap and rotor. All items are maintenance items. If good, check Cruiser54 tip #13 Renix Distributor indexing, as mentioned above.

 

https://comancheclub.com/topic/73472-13-–-renix-distributor-indexing/#comment-788499

[Gojira94]

18 minutes ago, AnotherOldJeepGuy said:

I'm guessing I would need another tool I don't have to test this.... correct?

 

In general, no. We're having you go through and check things to work backward to the cause. At a high level, running rich on an EFI system has 3 causes:

 

1) too much fuel pressure or faulty injectors leaking/ delivering more fuel than designed/ intended

2) extra air getting in somewhere that's not accounted for by the computer - the O2 sensor then sees a lean condition and the computer commands more fuel to compensate

3) Inadequate spark to burn the fuel delivered

[AnotherOldJeepGuy]

Gotcha, I was assuming I would need some kind of tool to check for "inadequate" spark.

[AnotherOldJeepGuy]

Vacuum plug back in place and hose tied off so it doesn't wander...

 

Pulled Dist Cap, the rotor only wiggled back and forth a tiny bit, maybe 1-2 degrees each way, very small.  Things looked pretty much the same shape as when installed in Jan.  Not pristine, but pretty clean.

 

Still no soap test, but I did give the manifold to exhaust donut the slight poke test.  It's not in very good shape, I am pretty sure if I upped the poke pressure I could cause it to crumble.  Maybe not 100% but for sure a notable amount.  Perhaps it would be worth just replacing it to rule that out?  I figure if I end up working on the exhaust manifold I'd be replacing that anyway.  I'd thing the cost is minimal for a new gasket.  Assuming the rusty bolts will let loose, I am wondering if the donut can be replaced without removing much else?  The pipe wanders around from there to the CAT, and I don't see any anchors, and I see maybe an inch of space between the pipe and the frame, would that be enough slack?

 

It's worth confessing that I have been reluctant to crawl around under the truck while leaving it running with no other human in the cab to cut the engine and hold the brakes.  But I have stuck my head under a little to try and locate from where the loud comes from but difficult to pinpoint.

[llhat]

2 hours ago, AnotherOldJeepGuy said:

On 5/30/2026 at 12:37 PM, Gojira94 said:

O2 sensor may be dead or contributing

I replaced this sensor back in Feb 2025 when I had an idle issue.  The issue turned out to be very dirty TB and IAC and was completely resolved after cleaning them and putting in a new TB gasket.  Of course it could still be contributing, just not because of age.

 

 

age may not be a factor on the O2 sensor...  many reported DOA 

[AnotherOldJeepGuy]

3 minutes ago, llhat said:

 

age may not be a factor on the O2 sensor...  many reported DOA 

Understood.  Good to know.  This one has been working from Feb 2025 though.  But I wish I still had to old one to swap back in, since the issue back then didn't turn out to be the sensor, just to rule out it has not failed.