Abrupt RPM drop while driving/coasting - 3FE

Joined
Sep 4, 2013
Messages
528
Location
Raleigh, NC
 
Sorry for the long post. I really believe in providing as much info up front. I appreciate you taking the time to help.
I drive a 91 FJ80 with the 3FE. I am desmogged completely (no EGR/Cats/Air Pump).

I started having a trifecta of issues last weekend while on my club's annual ride.
1) Belt broke
2) Alternator went bad (confirmed by Autozone)
3) Random RPM stutter -- story below...

Friday's ride went great. No issues or concerns of note. Spent about 5 hours out on trails.

Saturday's ride started with hearing what I thought was ticking. I ended up stopping on the trail and found that the belt driving my water pump (crank-wp-alt) was torn and was flapping against a mount for the old air pump.

We made it out of the trail and I had a couple of guys follow me out. While driving back (gravel road-not real fast), it had a sudden, abrupt drop in RPM and the CEL illuminated went back out immediately. I thought the belt had given. It did not, but we didn't want to take any chances, so one of the guys pulled me back to camp.

I swapped the belt with OEM and the next drive went fine for about 10 minutes, then sudden, random dropping of RPM's, some to the point of the CEL flash.

The alternator was not charging the battery, so we figured that was the problem. My next test drive of 10 minutes had the engine stall out three times; so I had it towed home.

Once home, I had a reman OEM ordered from Onur (shout out to Beno for the great service!) The swap went smoothly and I drove it around afterward. 10 minutes of slow driving was fine. Hopped on highway for 7 miles went fine. Exited off highway and within 2 miles of 35 mph, the stutter hit again.
I made it back home and changed my air filter. I was overdue and thought the gravel dust may have caused an issue. My next drive was similar to the last. Highway was fine out...45mph fine...no stutters...until about to exit highway on way back home (60mph).

Yesterday, I drove the truck to work (I had carpooled with my wife all last week) and had no issues until I was almost home at the end of the day, starting from a stop light. Two quick stutters, one flash of CEL and all was right again. This drive included sitting in bumper-to-bumper traffic, and a pickup line for my kid's camp--all fine.

So at this point, I am not sure what's next to test. I don't have a ton of tools at my disposal so some tests may take a little time. I have some and some limited knowledge. But I'm in a great club that helps me out a ton!

Let me know if you need more info around any topic. I'll try to reply quick.
Thanks for the help!
-Derek
 
Joined
Sep 4, 2013
Messages
528
Location
Raleigh, NC
 
I realized I neglected to CEL codes I pulled.
It is giving 41 and 51 which both point to TPS; but I have had those codes present for over a year.
 
Joined
Sep 4, 2013
Messages
528
Location
Raleigh, NC
 
Thanks for the link, Steve. I will read over it and start that route. I tried to do some work on the TPS to test it earlier this year but really struggled with it under the hood.
I will move forward testing and, if needed, replacing the TPS. The symptoms you describe in paragraph 1 are in line with mine.

I will post back an update when done.
-Derek
 
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