'88 engine problem back...runs rough sometimes...

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Jan 3, 2011
Westchester County, NY
Replaced the fuel pump and filter recently chasing this problem...ran better, but still rough..sometimes fine, most times misses at below 2000 RPM's...above 2000 everything seems to kick in, and runs fine. Sometimes it idles just fine, other times it misses...mostly misses. I'm thinking FPR at this point...all hoses checked and rechecked...this problem has been going on for 3 months with me replacing parts and trying to track it down...
Heading to mechanic next Tuesday for exhaust, but I hate to give up the problem to them.:mad:
Could it be the timing?
Code 41

Ok I'm definitely getting a Code 41; Throttle position sensor. I'm not sure what to do next...and BTW, where is the ECU?
Thanks boys.

Checked all the voltages per the FSM on the ECU and TPS..everything seems to be within range. Still very rough below 2,000 RPMs. I'm lost.
Could it be old CATS?

Check the EGR tube that insterts into the intake, if you have not already that is. I had very similar issues to yours and this fixed it. .
Disconnected TPS...Bingo!

I disconnected the TPS connector and ran the truck around the block. HOLY CRAP!
It's fixed! All these months, all that frustration, and it runs great without the TPS. Idles at 1000 RPM however, not 650 like it used to, but runs way way better.

So I guess a new TPS is required. Happy to buy and install that bugger; but what exactly does it do? How could it be screwing up my beloved so badly, running horribly below 2000 RPM's, sometimes fluctuating RPM's on it's own when I held it at 2000, missing like it's running on 4 cylinders or something.

Anyway, that seems to have been the culprit. I'll just run it with it disconnected for now, until I can order one in. Any suggestions or hints for install or adjustment?

(BTW, all the ohms readings were within tolerance, so I didn't think it was bad)

Whew, what an ordeal.
Remember that the TPS isn't something you can just slap on the throttle body and go. It requires an adjustment when installed by measuring resistance of the 4 terminals on the TPS itself, but in a certain order/combination. Also found in the manual, but I do believe I have seen that page scanned and posted here in the forum somewhere.

Even if it is within tolerance a weak return spring can also make it play havoc with the engine and the way it runs.

New TPS goes for around $100 or so.
Gotcha...I'm betting weak return spring.
Cleaning the carbon out of the throttle body and then adjusting the TPS per the FSM specs should solve your problem. The TPS is sensitive to adjust but was doable. Mine had the high idle problem at about 290,000 miles and doing this fixed it for me.

ECU is behind/under the glove box.

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