Hey, gurus,

I've seen a few posts about idle issues when cold, but this one I can't figure out. '92 F-150 5.0L, runs great, starts and idles fine when it's cold. After driving it for a bit and restarting, it idles between 2500 and 3000 rpm for the first 30 seconds or so, then comes back down to where it belongs.

I've cleaned the IAC, and it actually got worse (used to idle between 2000 and 2500.) If I unplug the IAC, it runs fine, just idles a little low. That tells me that the IAC is doing what it is supposed to and the EEC is intentionally cranking up the idle speed. But why?

 

Pull the codes to see what comes up. I'd also check the ECT sensor to make sure it is giving the correct signal for operating temperature.

 

Uh, yeah, good point. Shoulda thought of that. Anyway, I get no hard faults, and three in memory: 31, 32, 41.

From what I can find, 31 and 32 relate to low (or non-existant) EVP circuit voltage. I don't know what EVP stands for, but it is apparently EGR related. 41 relates to EGO, another acronym which is lost on me.

Any pointers would be greatly appreciated.

 

EVP is the EGR valve position sensor. EGO is oxygen sensor, but I don't think that either of these are your problems. Check fuel pressure and check for vacuum leaks. I'd also test the ECT sensor to make sure it is giving the correct output.

 

ECT, as in Engine Coolant Temperature? I haven't measured the voltage on it, but assuming (yeah, huh?) the temp guage is fed from the same sensor, it's the same as it ever was.

Probably won't be doing much else today; it's snowing. Thanks for the info.

 

it can be the temp or the o2 or Egr becauce they send a signal to the EEC and it will retard or advance the timing.

 

ECT is the engine coolant temperature sensor. The computer sensor is not the same as the guage sender. If you have a repair manual, you can check and see what the resistances should be on the sensor. I wouldn't think that the EGR would affect idle speed - if it did, the idle would drop or stall the engine, not raise the speed.

 

Check parameters on your HEGO sensor it may be finished. They do "burn down" over time & fault operating under Op Temps.

All those D-Codes can be a sequential thread based on a single fault, HEGO is involved with EGR & EVR

FWIW, EGO = Exhaust Gas Oxygen & HEGO = Heated Exhaust Gas Oxygen meaning your O2 sensor reads both COT & HOT values then shunting signal as per your temp switch readout it's why there are multiple leads on the O2 sensor & also why some sys' have upstream & downstream O2 sensors.

EGO/ HEGO & EVR values determine what IAC will do. The time spent by IAC seeking correct function will increase idle until it gets a read & adjusts itself.

Higher hot idle speeds that calm down is only a "failsafe mode" from the RAM so engine runs long enough for ECM to act.

BTW COT= Cold Op Temp & HOT = Hot Op Temp or the high/ low parameters of what ever system/ function you're checking.

After making ammendments/ repairs don't forget to dump your D- codes or youy will continue to experience the symptom or the corrective action until the fault works out of the program, like 45 47 start cycles later.

FBp

 

The O2 sensor has no effect on EGR valve function. The O2 sensor has a heating element in it to get it to operating temp faster. It doesn't change readout based on engine temp. Once the O2 sensor is up to operating temp, the computer starts reading the signal from it. At start up, where the high idle is, the signal from the O2 sensor is not used anyway. There are many more factors that contribute to what the IAC does. If you haven't seen this website, it's an excellent read: http://fordfuelinjection.com . If the engine is hotter than operating temperature, the computer will kick the idle speed a little bit, but not as high as 2500-3000 rpm. On a cold start, that would be normal, but on a hot start, it shouldn't do that. The ECM begins functioning as soon as the engine starts, so there is no lag time waiting for it to start to act. The high idle is for cold start warm up, which shouldn't happen when the truck is warm. I would reset the codes and unplug the computer so it can relearn the operating ranges of all the sensors after fixing the problem. However, if there is a problem, and the problem gets corrected, you shouldn't experience the same problem anymore, but it just won't be optimal. Does the engine idle correctly if the kick the throttle before during the 30 seconds of the high idle? If it does, I'd say that the computer is kicking up the idle speed for some reason, probably due to the signal from one of the temperature sensors, as it would on a cold start.

 

The good news is that it isn't snowing anymore, so I can get out and play with it some more today. I'll take a peek at the link you send, EPN. And to answer your question, no, you can't kick the idle down. You just have to wait it out or drive - but the latter tends to eat some clutch if you're in a parking lot or something :-)

 

That doesn't sound like the computer is running the idle up. You said you cleaned the IAC, right? What does it do if you unplug the IAC and then start it?

 

With the IAC unplugged it runs fine, but idles a bit low. It will stall if you load the PS pump or flip on the AC.

I haven't found the ECT to take any measurements - I didn't spring the $200 for the emissions manual, and that's the one that has all the sensors in it :-( If you can point me to it, I'll at least be able to see if it varies with temperature.

 

Just a regular Haynes manual should give you enough information to check the ECT. It's right next to the thermostat housing - to the left as you are looking at the engine. Since it does not jump the idle speed when the iac is unplugged, the computer is telling the iac to open up. I'd just double check that all the intake manifold bolts are tight, and the throttle body and iac bolts are tight as well, and that there are no vacuum leaks that you might be missing. I'd also double check that the TPS is giving the correct output to the computer.

 

OK, progress (maybe.) You got me moving with the vacuum comments.

Anyway, I either have an intermittent or small leak, or a bad thingy which I hope you can help me identify (haven't gone downtown to get that Haynes manual yet!) If I touch this thing, I can get the idle to hunt. It's a sensor (measuring vacuum, maybe?) of some sort, hanging off a mounting plate right in front of the EGR, to the left of the throttle body. The bottom of it is teed into a vacuum line.

So whatever it is, I happened to tap it while I was hunting vacuum leaks, and lo, the idle went up a few hundred RPM. Give it a wiggle, and idle comes back down. The vacuum lines are pretty ratty, so I'll replace those and see what happens. If you can tell me what that sensor is, I'll see if that's a cheapy and replace it too.

 

From the description, it sounds like the canister purge valve. I think they are either normally opened, or normally closed. The vacuum line from it should go to a box on the frame, and the other side should go to the throttle body. That sounds like it may be where you're getting problems. Can you plug the vacuum port at the throttle body and see if that makes a difference?