Guess I've finally reached the point of getting tired of banging my head against the wall on this.  I've got a 1990 RRC that has a weird stumbling problem.  It used to do this only occasionally now it is getting more severe and frequent when it does happen.  This stumbling I'm describing seems to happen after it gets warmed up.  Only happens when engine is under load (pulling a hill, or on the highway).  When it stumbles it quits firing but has always taken back off without stalling.

When I first got the truck (year and a half ago) it had a TPS sensor code.  This made sense so I replaced it.  No luck with problem.  All codes gone, no check engine light?

Two months ago I was going through the 120,000 mi service. 

New Plugs, new Magnecors, fuel filter, air filter, replaced all the vaccum lines and the T piece, flame trap, rotor, distributor cap, cleaned all grounds I could find, replaced alternator (failed a week after other service), changed all fluids, battery tests good, adjusted timing (seems happiest at 7degrees BTDC).

Suspected MAF so I unplugged it out of curiosity.  Ran rich exahst smelled bad,(shortening lifespan of cats) took it for a spin on a known problem hill, no stumbling, got on the highway, no stumbling, tried hill again, no stumbling.  Had a buddy with an extra one so I tried a different MAF.  Same problem. Makes me think it may be something not present when truck is in "limp home mode"?

This last weekend headed out to enjoy the sun, southbound on I-5 stumbling so bad I got off the highway.  Stopped to let the dogs have a break, truck shut off for 10 minutes, the whole way home not a single problem?

Would a bad amplifier on the distributor do this?

Bad Coil?

Fuel pressure regulator?

Fuel pump?

Don't want to keep throwing parts at it.  Everything else I've done I'm chalking up to maintenance. 

Any testing or ideas would be very helpful.

rotor appears to be an "S" type

Cap ???

Brett,

I think you are on the right track. I had the same situation about 8 months ago and just started the troubleshooting process in the same way that you have here. Everything that has been listed is spot on. At the very end of my process, after new plugs, new wires, new cap, new rotor, new coil and new fuel filter, I was convinced it was the amplifier. I placed the order on line and waited for it to be shipped. While I was waiting I went to remove the amplifier and then had second thoughts about doing it before I had the new part. However, i did clean up the connections on the amplifier and the wire ends coming in with electronic contact cleaner. Put the wires back on the amplifier and bam...problem gone. I zip tied the wires down snug on the amp and have not had a problem since then. The new amplifier is still in the box in my parts bin.

Might be something for you to try as well.

Good luck...you will get it fixed.

Dustin

Update....

Changed out the plugs to the champions this morning.  Gapped them per the manual.  Fired it up.  The old Bosch Platinums were all in pretty rough shape.  Carboned up.  Noticed right away a smother idle.  Took it out for a test spin.  Ran great....for awhile.  More power, better kickdown response, warmed up to full operating temp, still running good.  Did one more lap on a known problem hill (long steady grade of a couple of miles) and the stumbling reared it's ugly head.  Same as before, only happens while under load. 

My wife is running around in it today while I do the brakes on her disco.  Tonight I'll pull some plugs and see what shape they are in after some highway miles.  Removing the old plugs, all the cylinders were bad, some had more carbon than others all had some white residue on the electodes.  Guess that's good news that one didn't look lots different (steam cleaned for instance).  I still have them in the layout they were removed from the engine. Pics of two examples.

After I look at the plugs tonight I am also going to check the connection on the amp.  Clean the contacts and plug it back in.

I was driving it pretty hard during the test this morning.  When it stumbles it completely cuts off.  Maybe up to half a second.  Seems like the harder I'm driving the longer the cutoff is.  The tach does drop when this happens, then it takes back off. 

I'll post a pic of the plugs once I look at it tonight.

What should be the next step?

Not sure on the cats?  No rattling around when I pound on them.  The entire exhaust looks to have been replaced at some time by the PO?  Looks to be the two pin ignition amp.  Where to tuck my greasy fingers next.