sporadic fast idle
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sporadic fast idle
The truck is a 1990 F250, 4wd, 5 speed transmission, 5.8 liter (45K on Ford rebuilt engine)which has developed this problem: after reaching operating temperature, it will, on occasion, idle at 1500 rpm rather than the normal 750. If the engine is stopped and restarted, it usually returns to the slower idle, at least for a while. It doesn't do it every time,and it occurs at varying outside air temperatures ( we are in the desert, temperatures can vary from 15 to 105 degrees). We have checked the pcv and vacuum hoses, we have replaced the iac even though the ohms were in spec on the one in the truck. We don't have a diagnostic tool, but had a mechanic check for error codes, and came up empty. The truck is used in rough country, doing hard work on our ranch and we try and take good care of it. Any help will be appreciated.
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Post Fiend
Joined: Nov 2002
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From: Memphis, TN
Fleet Mechanic
Joined: Aug 2001
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From: Round Rock, TX
sporadic fast idle
This doesn't help you, but I'm having the exact same problem with my '92 351W, and the only code is the MLPS code. I've got a vacuum pump, my multimeter, my Ford service CD, and a lot of patience, so I'll be trying to eliminate various sensors over the next few days. I'll let you know if I come up with anything.
Fleet Mechanic
Joined: Aug 2001
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From: Round Rock, TX
sporadic fast idle
I just adjusted the TPS again today, so we'll see how it acts. I had it reading 0.990 V at rest, but I noticed that it didn't always go back to 0.990; it would sometimes stop just above 1.000 V. So, I backed it off so that it's at 0.975, and when it doesn't come to rest perfectly, it's still below 1.000 V.
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sporadic fast idle
Just a thought. Have you guys checked you t-stats. I'm by know means a mechanic but on cold days the stat will stay closed and rpms are high until engine reaches temp. once engine reaches temp stat opens and rpms drop. If the stat is getting stuck closed wont it tell engine to rev to get to proper temp. Possible sticky t-stat? Maybe? Just guessing.
Definitely curious to see what you guys find.
Good Luck!
Definitely curious to see what you guys find.
Good Luck!
Fleet Mechanic
Joined: Aug 2001
Posts: 1,526
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From: Round Rock, TX
sporadic fast idle
My thermostat is good, and I verified the voltage output of the ECT sensor, so the computer is getting the correct temperature data.
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Fleet Mechanic
Joined: Aug 2001
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From: Round Rock, TX
sporadic fast idle
Knocking the TPS back like I described above fixed my problem, and the problem sounds exactly like yours. Give it a shot; it only takes about 10 minutes, and if you're careful, you won't need another throttle body gasket.