man_bear

Good evening everyone,

Bought me a '93 F150 XLT 4wd with the 302 back in March and ever since then have been chasing a high idle on the engine. What is currently going on is after cranking the truck it does it's normal rev up to 2500 rpms and then slowly goes down, but instead of around 700 rpms it will just hang out around 1500 rpms. If you press the accelerator it will rev like normal, except it will hang out between 2000 and 2500 rpms, then slowly go back down to 1500 rpms. When you turn on the A/C it will bring down the idle rpms to 1300 but it still acts the same when you press the accelerator. Now if you unplug the IAC, it will still sit at 1500 rpms but when you press the accelerator instead of hanging out over 2000 rpms it will come right back down to around 1500.

So here is a little backstory/what I have tried so far:

The guy I bought it from had unplugged the TPS due to the high idle issue and that obviously didn't solve the issue. When it was parked at a buddies house for some other repairs (slave cylinder went out on it about an hour after I bought it) he checked the TPS that was on it and it wasn't showing proper voltage so went ahead and replaced it. Since then I have swapped the IAC with an extra one a coworker had (see picture, the one on the left was what was on the truck and the one on the right is what coworker gave me). I have checked it for a possible vacuum leak but when I plug in a vacuum gauge it is showing 20 pounds of vacuum at idle. Same coworker that gave me the IAC said that it could be the MAP sensor so I went ahead and bought a new one of those and put it on this evening to no change. Something I had found online was a way to help the computer learn proper idle by using a couple of pencils to block the two little holes in the butterfly valves on the throttle body, which with the IAC plugged in brings down the idle to 1100 and unplugged it sounds lower but I didn't have anyone nearby to see what it dropped down to. As soon as you remove the pencils it will idle right back up to around 1500.

I apologize for my first post being a long winded cry for help but I have been scratching my head quiet a bit on this and would appreciate any help!


Old IAC on the left and newer one on the right.

RollTideTony

I think we’ve got the same problem 🤔

bad packet

Coolant temp sensor is a common cause of high idle... basically the ECU is putting the truck in high idle cause it thinks its cold. Sounds like you've already hit on the likely suspects, TPS sensor, IAC, vacuum leak. You could also try fogging around the vacuum lines to see if you have small leaks. It would not be surprising if a 28yr old truck has some cracked lines. How about codes? MAF issue perhaps?

RollTideTony

Ordered MAF, IAC valve, and already installed new TPS. Figured I’d eliminate those right out of the gate and I’ve already replaced all vacuum lines during the install. This Ol boys gotta get goin

Steve83

Throwing part$ at a problem doesn't eliminate anything - it ADDS to the unknowns. If you suspect a problem with any part, TEST IT without disturbing it or anything else any more than absolutely necessary. Get a Haynes manual, and READ IT cover-to-cover at least once so you know what's in it and where to find things. Then use any decent digital multimeter (even a $20 cheapo is good enough for most automotive work, but expect to pay $50-80 for a good one) to test any suspect sensor or actuator. It's cheaper, easier, quicker, & safer than shotgunning random parts at it.

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If a part fails a published test, buy the BEST replacement available - not just the cheapest or most-convenient. And TEST the new replacement (preferably BEFORE installation) - if it fails the test; either you're not testing them right and the original is OK, or (and this DOES happen frequently) you bought a bad new part. Either way, there's no point installing the new one until it PASSES the same test that the current part failed.

primalcompletions@gm

Check the timing see if the past owner might have advanced it doe to possibly a low idle problem they might have had. Usually this would be done by turning the distributor a little by hand. Could have vacuum lines blocked or reversed. Maybe the egr valve is not working. Bad smog pump canister, purge charcoal canister. Idling high at a constant,8 I i would start by checking the timing though. If nothing seems to work check the ambient air filter. Sounds strange yes I know but have had these cause idling issues. Usually on newer vehicles.