Interesting problem I am having, helping a friend get an older motorhome running.

Initial complaint was bad fuel pump - so before dropping a huge metal tank, I checked the pressure - and all is good (around 30 at idle, remove vacuum from reg and it jumps to around 45ish).

Found some other issues - broken plug wire, corrosion in dist cap, 2 different brands of plugs...overall lack of maintenance in a while.

Based on the way things looked, we replaced the following items:
Cap
Rotor
Wires
Plugs
Ignition Module
Coil
EGR Valve (corrosion)

Now, it seems to be faltering and missing at idle - if I give it a little throttle, it gets a little better - and if I get it over 2500-3000 RPM, smooths out fine - so a bit befuddled here.

There was a coolant leak on a small hose just in front of the thermostat housing, so it had been dumping coolant everywhere - not sure if maybe it got into the distributor and would be causing issues? I am currently soaking the distributor bore in kroil to get it loose so I can verify timing - if it needs to be replaced, that would be the time to do it.

I've seen comments about plug wire routing (especially 7 & 8), so I made sure they are well separated.

Trying to dredge up all the old tricks that I haven't used in forever, since I usually am working on newer stuff - anyone seen similar, and might have an idea of what I am missing (pun intended)?

 

Could easily be a vacuum leak or stale gas. Or not.

Have you run an OBD1 PCM Quick Test to see if the antique PCM has any fault codes?

 

Was it missing before doing the work? Same type and rpm range of missing? Could be a bad new part you put on.

 

Haven't run the PCM codes yet; got pulled off that one while in the midst of a 2014 Escape transmission replacement...I don't recommend that one.

It did have hesitancy prior; attributed that to some extent to a broken plug wire - but now not sure. With the coolant leak spraying in the vicinity of the distributor, was wondering if there may have been water intrusion/damage there - but also not sure if there's really enough "electronics" in there, beyond the simple pickup?

Customer states they were questioning IAC as well, since idle was erratic previously - I didn't have that info until late yesterday.

Thanks for the additional items to look at - will be back at it later this week; not a rush job, but also don't want it to drag on forever.

 

Been a while, but since I have gotten this resolved, wanted to post findings, just in case it helps the next person out...

We went through all the typical culprits, checking for vacuum leaks (did find that the EGR solenoid was leaking thru, barely enough to move the valve, but even when disconnected, didn't resolve the issue).

Ran ECM KOEO codes, had a EGR closed and SPOUT error - but didn't find anything other than what appeared to be a voltage ref error.

To the Google I went - and finally stumbled across this video that was a mimic to the issue we were having.


This guy had a little better tracking tool, but really narrowed down the ref voltage issue. Based on his video and findings, I pulled the PCM out (of course it's mounted right behind the dang brake pedal) and popped the cover off... Sure enough, a blown capacitor on the board; just like shown in the video.

I didn't want to mess around with finding a good source of capacitors, and trying to exercise my mad soldering skillz - so I took a chance and sent it off for repair to these guys - https://modulemechanics.com/product/...-computer-oem/
Sent it out the Monday before Thanksgiving, and had it back today; pretty impressive service over the holiday.

Plugged the repaired PCM back in and buttoned everything up - and this thing runs like a top now - no more surging/misfiring, no backfiring under load, etc - so definitely a valid repair.

Customer is happy, and so am I - parts that were replaced were definitely showing their age, so customer isn't upset about having that bill, either.

So if you're experiencing a similar issue - check the PCM for a physical issue - sometimes it's the weird things that will get you...

 

didn't replace any parts other than taking out the wiring harness and fixing a Short in it. Everything else is the same.