hi i just finished installing a junkyard 5.8l engine in my 94 f250, the engine had been sitting for awhile but runs really smooth with 38k miles!

anyhow, it will run well, warm up then kick up to the 1850rpm idle, ive been researching

for the hell of it took the iac off the old motor (old motor ran perfect, idled perfect, just leaked a lot of oil from the front main seal)

no change, no change unplugging the iac, unplugging the map sensor it idles normal but chugs black smoke

searched like hell for a vac leak using propane, starting fluid, checked the intake manifold for leaks, no change in idle, gone through a whole can of ether, pulled each line off one by one and checked for leaks with the engine not running, all line circuits hold vaccume brake booster is ok (clamped line shut, no change)

pulled the 2 wire coolant sensor out of the old engine, still no change after putting it into the new motor

took the tps out of the old motor, still no change

took the whole throttle body off the old motor and put a new gasket & installed on the new motor, no change (did this because the tps wouldnt come out easy)

one thing i do have is an ABS light on for no reason, as i said, goes into normal warmup mode then after 5 mins goes crazy up to almost 2k rpm idle

any other ideas? someone told me a speed sensor will cause high idle probs? but the speedo works fine, though the trans wont really downshift

also, guessing i should take out the other ect sensor (3 wire one) and try swapping that as well?

im at my wits end, ive spent about 4 hours going over every inch of this engine compt., ive let the motor run and run to get the air out of the coolant, so im positive its all purged

thanks for any tips or hints!

 

if theres no change with idle when you unplugged the iac/iab, its bad, at least thats the way i understand whats been said on fte
ask SUBFORD

 

Pull the codes, if there is a sensor problem something should show up and if you have already changed that part then maybe there is a wiring problem(pinched/broken wire) or the issus is something that is not under computer control.. like a sticking throttle cable or a gasket leak.

 

Check for vacuum leaks

 

I do not know if this will help you, but my truck idle's at around 2000 rpms. When I got it the seller said that was normal, bull----. I replaced ever sensor, upper intake gasket, vaccuum hoses, check codes and it stills does it. I have been all over the internet and find people with the same problem but no answers. What I discovered about a month after I got it, if I hit the gas (rev the motor for a brief second) right when the motor starts running, the idle goes down to normal about 98% of the time. If it still idle's high I just repeat the above step. I been doing this for 2 years and 15,000 miles. I think mine is the computer, but I am to cheap to replace it. I figure I just keep driven it until it breaks down. It cound also be one of the new sensors I replaced was bad when I got them.(autozone brand)

 

I would point at either a vacuum leak or that you need to reset the throttle screw (wich i must say, is a pain, and i don't remember the procedure).

 

The hard idle should be set at 650rpm, that is with the idle air bypass valve disconnected. At the same time the Tps should be set between .75 and 1vdc, .99 for performance. Then reconnect the Idle air bypass valve and it should idle correctly.

You may have to do the hard idle a couple of times because changing the throttle and TPS will each effect idle, so a little patients is in order. You may have to reconnect the IAB to get it to stabilize at times.

When done disconnect the battery negative side for 10 minutes and reconnect.

I am going from memory, but have the complete procedure written down somewhere.

 

sounds good, im going to set the hard idle tomorrow, i tried interchanging the other coolant sensor but no change

idles normal for cold startup (like 1400rpm) then after fully warm it goes up to 1700 or so and stays there

im positive there are no air leaks

so im guessing the idle reset is in order

i can force the butterfly a little farther and bring the idle down a little, but then the iac kicks it back up a moment later


thanks everyone for all the replies!

 

really quick, how do i adjust the tps? it looks impossible the way its on down there unless i remove the intake again

 

Most people use a volt meter and pierce the wires with a needle and read the setting with a volt meter.

I purchased a meter that plugs in the connector and is very simple to do. Interactive Systems & Technologies Mass Air Home Page is where i purchased mine and they send a very good write up to help you. It's excellent if you have a few of these vehicles because this is a frequent problem.

 

That is an urban myth. The TPS closed value can be anywhere between 0.6 to 1.0 VDC. The PCM is always looking for the minimum registered value of the TPS signal then adjusts from there. There is nothing to be gained by adjusting your TPS as long as the closed value is in that range.

What does cause problems is a varying signal back to the PCM caused by dirt/corrosion on the wiper inside the TPS itself. The best advice to measure the signal output with a meter while the key is in the ON position. If you see any dips or inconsistencies, replace the TPS.

Please read this for reference: Ford Fuel Injection » Throttle Position Sensor (TPS)

 

really quick, how do i adjust the tps? it looks impossible the way its on down there unless i remove the intake again
On my 302 I had to take the throttle body off to adjust the tps.

 

Use an offset screw driver.

 

Not meaning to hijack your thread, but can anyone explain why this procedure works with bringing down the idle. I've been doing the same thing for years (I'm the original owner) with my truck. The only problem with this method is that there isn't enough dash pot for changing gears at slow speeds. This seems to fool the computer into keeping the idle from running too high. I often wondered if there is a bug in the Look Up Table programming.

 

Ed what does this mean? isn't enough dash pot for changing gears at slow speeds. I looked all over the internet and can not find how to fix it. I hope someone answers this. That is the only problem I have with my truck. My truck runs strong from idle all the way up to 5,500 rpms and that with the 35 inch tires. I do not baby it.