Stalls At Rapid RPM Decrease, Foot-Off Accelerator, Otherwise Runs Fine

[AnotherOldJeepGuy]

I originally tagged on to this post by @BlueDove91, "Intermittent Stalling/Dying While Otherwise Driving Fine...Restarts, Drives, Randomly Dies Again" because of similarity.  This was 1/6/25.  I had no expectation how long my "tag on" would last!  Over time I think my issue has diverged enough that, in hindsight, it should have probably been it's own thread.  Too late to start this issue 1/6/25, but I think I should re-title, summarize, and move forward with a more appropriate title.  So I will with this post, and with the new title "Stalls At Rapid RPM Decrease, Foot-Off Accelerator, Otherwise Runs Fine".

The problem description may be better stated as "any time I take my foot off the pedal and let the RPMs fall to what should be idle, it instead falls below that RPM and stalls" (which does differ from the OP description).  However I can start up any time, and before any acceleration, and it will idle perfectly as long as I let it.  So it is only the RPM decrease by taking my foot off the pedal that causes the stall.  Whatever is supposed to limit the minimum RPM is not working, or not working fast enough.

 

Over the last two months, these things below have been suggested, and done, and I am sure have been a benefit overall as well as educational, but have not fixed the problem.

 

- Searched for vacuum leaks.  Visual inspection was not able to see any cracks or breaks or things no longer connected.  Vacuum leaks were found with water spray over just about everything in the engine compartment.  Leaks were located at the intake manifold to engine bolt up where several bolt were loose.  When sprayed in that area engine RPMs fell immediately.  After tightening these loose bolts, the water test was repeated and no longer showed the signs, RPM was unchanged.
- O2 Sensor Replaced with new part.
- Throttle Position Sensor Replaced with new part.
- Passed EGR Test as described by https://comancheclub.com/topic/73480-21-–-renix-egr-valve-test/#comment-769327
- Removed, cleaned and re-installed the Idle Air Control (IAC) which seemed to help at first, but that was a red herring.  It helped when I revved the engine in the garage right after doing this, but failure returned very quickly (about 3 starts later).  Update: Also cleaned what I could get to in the TB short of removing it for a more thorough cleaning.

 

Pending
- Use contact cleaner on electrical connections when I get some DeoxIT or CRC 05103 QD Electronic Cleaner sprays.  As @89 MJ  said, "I have my doubts it will fix it, but we can hope so!"  I will post what happens when I do this, or any other steps taken so that if this ever gets fixed, that will be captured here.

 

Edited Thursday at 02:48 AM by AnotherOldJeepGuy
Update details of IAC cleaning to including TB as well.

[Gojira94]

When the throttle snaps shut at RPMs above 12-1500 or so, here's what happens:

1. MAP drops to lowest possible (15-25kPA), engine braking starts. A LOT of timing advance is needed here due to the leaning of the mixture (see #2).
2. flip side of MAP drop is vacuum increase, pulls hardest on FPR diaphragm, more than idle, part throttle or WOT). This equals lowest possible fuel pressure.
3. unknown if Renix calibrations have a DFCO (Deceleration Fuel Cut-Off) or other similar routine, I'd guess there is some form of one to reduce fueling in decel.
4. IAC opens more (pintle retracts) some number of extra steps to prevent stalling on closed throttle (TPS) with RPMs above ECU's target idle value in gear.

Things to check related to drivability/ stalling will be if there is adequate fuel under decel, if timing is adequate and whether the IAC can provide the bypass air needed. Highly unlikely that the ECU-commanded extra spark advance will be changed, though it's possible that indexing of the distributor prevents commanded advance from being actually on the rotor tip (complete loss of spark at commanded degrees of advance). Other is vacuum leaks adding more air than commanded spark advance can burn with leaner than expected condition (inadequate fuel for condition). You're pretty sure vacuum leaks are out of the equation. Easiest thing to verify is fuel pressure at the rail. Distributor index check is obviously more effort. A check and cleaning of the IAC motor and housing never hurts. (you already did this)

[AnotherOldJeepGuy]

2 hours ago, Gojira94 said:

You're pretty sure vacuum leaks are out of the equation

Thanks.  I have done all I know how to find and eliminate any vacuum leaks, but my ASTM Degree (Advanced Shade Tree Mechanic) degree is by no means professional.    A more thorough cleaning of the IAC / TB is on the to do list since initial cleaning of the IAC did seem to help a little.  I don't have any equipment to measure fuel pressure in the failing condition or otherwise, or the expertise to do some of the other things you mentioned, at least not yet.  I am planning new plugs, cables, Dist Cap in the near future, although with it running fine other than this one specific issue it makes me feel like these items are not that bad, purely intuition at the moment.

 

I am doing a band-aid experiment right now.  I am sure the throttle stop is adjustable although I can't see what is needed (flat-head, Phillips, torx, allen,?), but rather than screw with that adjustment directly (pun intended) I have changed the stop point in a much more reversible way.  I don't love this fix, but I am OK with the experiment.  This new stop point doesn't really sound like it is idling any faster, and revving in the driveway is not stalling (yet).  So I will go with this for a day or two, or until it fails if sooner, to at least get more diagnostic information.

[Gojira94]

The IAC will try to compensate if the throttle blade is a little too far closed at idle, or too much open. It can be defeated and not be able to open or close off enough air if the throttle stop is too far either way but that doesn't look like an issue, though stalling with throttle snapping closed could indicate it's a little too closed and the IAC can't increase bypass air quickly enough to keep it from stalling.

 

I don't recall if the 1990 fuel rail has a Schrader valve. We used to tape a fuel pressure gauge to the windshield back in the day lol. I can see that being difficult with the hood on and shut far enough to drive safely.

 

When you do the cap and rotor, cut a window on the old cap an rotate the engine to TDC on #1 and see where the rotor tip is. A complete write-up on that is in Cruiser54's Renix tips Index under the Modifications & Repairs sub-forum. Here's mine before and after:

 

 

 

[AnotherOldJeepGuy]

Good Info.  I don't know that I have what is needed to move the engine to TDC, but perhaps after I look over the info from Cruiser54's post I'll know that.  Once I give my test case a couple days to see what I learn, I'll move on to the next step.

 

The fuel rail does have a release valve although I don't have anything to attach to it.  I can see that taping a gauge to the windshield could become problematic

[Ωhm]

2 hours ago, AnotherOldJeepGuy said:

new plugs, cables, Dist Cap in the near future

 

Maintenance items.

[pizzaman09]

The other thing that comes to mind, is if you have a cracked exhaust manifold.  That could tell the O2 sensor the incorrect info.  To check this you could unplug the O2 sensor and see if the sudden off throttle issue remains. 

 

I also wonder if the MAP or throttle position sensor are not working correctly.  Have you checked to see if the throttle position sensor needs to be reclocked to get it to be at the right reading when the throttle is closed?  There is a cruiser tip for checking it with a multi meter.

 

https://comancheclub.com/topic/73467-8-–-tps-testing-and-adjusting/

[AnotherOldJeepGuy]

@pizzaman09  I'll try disconnecting the O2S and see if things work differently after my temporary throttle stop adjustment test runs it's course for a couple days.  At the moment that is working perfectly as best I can tell.  Which should be a clue rather than a fix, but if all else fails....

 

So, if disconnecting the O2S happens to makes it run "better", what the heck is the O2S for then!?   Seems like disconnecting it would cause trouble if it is there for any useful purpose, so I am confused!

 

I put on a new TPS just because I got one at the Adv Auto liquidation for about $4, but that didn't change anything.  Although I suppose that could just mean that the new one is as far out as the original, so I'll take a look at that tip.  I have not done anything to check the TPS other than just replace it.

[Eagle_SX4]

2 minutes ago, AnotherOldJeepGuy said:

I have not done anything to check the TPS other than just replace it.

Check this for adjustment procedure.

 

 

 

[pizzaman09]

1 minute ago, AnotherOldJeepGuy said:

@pizzaman09  I'll try disconnecting the O2S and see if things work differently after my temporary throttle stop adjustment test runs it's course for a couple days.  At the moment that is working perfectly as best I can tell.  Which should be a clue rather than a fix, but if all else fails....

 

So, if disconnecting the O2S happens to makes it run "better", what the heck is the O2S for then!?   Seems like disconnecting it would cause trouble if it is there for any useful purpose, so I am confused!

 

I put on a new TPS just because I got one at the Adv Auto liquidation for about $4, but that didn't change anything.  Although I suppose that could just mean that the new one is as far out as the original, so I'll take a look at that tip.  I have not done anything to check the TPS other than just replace it.

Unplugging the O2 sensor would put the ECU into a lookup table mode that just makes a safe guess as to how much fuel to supply if it's not getting a signal from the O2.  I'm guessing it would be very rich and not allow for good fuel economy.  The reason an O2 sensor could get a bad reading is an air leak upstream to it will make the signal not correct.

[llhat]

4 hours ago, AnotherOldJeepGuy said:

A more thorough cleaning of the IAC / TB is on the to do list since initial cleaning of the IAC did seem to help a little. 

in your prior post you showed a pic of the IAC pintle with the buildup of 'stuff' on it... you did not though IIRC state cleaning of the orifice in the throttle body, so yes, this could be at least another trial...

[AnotherOldJeepGuy]

2 hours ago, llhat said:

in your prior post you showed a pic of the IAC pintle with the buildup of 'stuff' on it... you did not though IIRC state cleaning of the orifice in the throttle body, so yes, this could be at least another trial...

Yeah I forgot to include that I also cleaned the TB as best I could short of removing it for a more thorough cleaning, but still the next time I'll be taking it off so that I can get to it better.  I went ahead and updated that post just for completeness.  Good catch.

[AnotherOldJeepGuy]

After 3 or 4 test drives, my throttle stop experiment sure appears to perfectly solve my issue.  Just noting that, not suggesting it is the appropriate (by the book) fix.  Since it only has any effect at min throttle, anything above that is working just the same as before, for what it's worth.

 

I do still intend to try the TPS adjustment and additional TB cleaning (a new TB gasket is on order) to see if I can get this working correctly and also per spec without the throttle stop modification.  I am making the determination of "perfectly solve" just by listening to my RPM and mentally noting they sound good at idle, as well as I no longer stall when I take my foot off the accelerator.  In other words I don't notice that I have induced any higher accelerator-off-pedal idle (other than the idle being higher than when it stalls ).  My question is, how would I go about seeing what the idle RPM actually is, given I don't have a Tachometer, adding one is probably overkill, and getting an OBD1 reader is not currently on the to-purchase list.