High Idle Blues 1994 4.0

[mailboxman]

need some help guys--i have exhausted the rangerstation forum for responses and was not able to fix my idle--
Replaced:
MAF
IAC
TPS
ECT
Spark Plugs
Wires
PCV
Have reset computer multiple times--bought a new thermostat that I will be installing tonight-and also bought another can of carb cleaner (this is the third can) that I will search again for vacuum leaks.

My cold idle is 1500+ and after warmup goes to 1200rpm

Cant find a vacuum leak and vacuum always pulls 20 on the gauge.
If I unplug the IAC my idle goes down to about 600 rpm but presents other drivability issues such as wont idle when cold.

If I unplug and block the pcv valve it idles perfectly-but i am worried about sludge in the engine.

If it were a vacuum leak, would idle go down so low when I unplug IAC???

And why does it idle so good when I block the pcv hose???

If anyone can help it sure would be appreciated

thanks

 

[jd4242]

does it run high in gear?have you checked your fpr?or lower intake bolts?

 

[mailboxman]

Thanks for the response JD--not real sure what the fpr is??
It does run high in gear--most noticable when slowing down in 4th (manual trans) and it starts bucking a little--I torqued all manifold bolts both upper and lower and they were already tight--once again, i have sprayed so much carb cleaner around that manifold looking for leaks it will probly start pealing the paint!!

 

[jd4242]

The fuel pressure regulator it pulls vac from the tree also.unplug that and plug it with your finger and see if rpms drop.I had that problem it would idle high and was losing power

 

[mailboxman]

Thanks for the advice JD--but I dont see the fuel pressure regulator?? Are you sure a 1994 has a vacuum operated regulator?? I did disconnect and plug all vacuum lines including brake booster and pcv --idle was good with all blocked--i then started reconnecting vacuum lines and idle was good until i unplugged the pcv -- idle went right up!!!

This is a long shot--but--i noticed today that my heater was not blowing really hot--it was blowing warm but not warm enough--if after changing all my sensors, and if the engine has never reached its max operating temp, maybe computer has never reset correctly???? I was going to change the thermostat tonight but we have a major storm blowing in and I dont want trouble in the morning so my thermo will have to wait till weekend.

 

[jd4242]

You have one,its on the left side of the intake and bolts to the fuel rail.it pulls vac of the vac tree.how long did you disconnect the battery for?

 

[mailboxman]

Did this after each new part was added--sometimes 10-15 minutes--a couple of times overnight--i also always pump the breaks a couple of times just to make sure all the power is out of the electrical system---I have been on a lot of forums and it seems that is is a common prob on a lot of 4.0's but once again nobody ever posts a completion---It seems to me that the idle rpms 1500 cold and 1200 hot are a key to solving this???

I am baffled--looking forward to the thermostat change (not doing it but the results afterwards) hope this could be it.

 

[Potomac30]

Is the hose hooked up from the air intake (downstream of the MAF) to the oil filler neck?

 

[mailboxman]

Yes Potomac--double checked it at lunch today--i can blow right through it--This truck has 165,000 miles on it and I have been babying it since I got it--it really is a nice truck--when i pulled out onto the highway today, i got on it a little and was taching to 4000 before shifting--noticed that my cel came on but went off after i left off the gas??? I will check stored codes again and see what it tells me.

Why would you mention the intake breather hose? Just curios how this could affect anything??

 

[jd4242]

if one side is off then the other side would be just sucking air in,its a close loop sys.you said you pulled each line off one at a time a blocked it right?is your butterfly close all the way in your tb?have you checked your fuel pressure at the fuel rail?

 

[mailboxman]

very interesting JD--I actually pulled all the vac lines off and blocked all the ports on the vac intakes and ran the power brake hose to the pcv inlet--idle was fine--i then connected each vacuum line up one at a time--idle remained good--then I blocked the pcv line with a spark plug and connected the vacuum brake booster--still fine--i then pulled the spark plug out and reconnected the pcv valve and the idle climbed right back up???

I checked the butterfly opening by screwing the screw out until i could get a thin feeler gauge between the stop and the screw--then turned the screw in 1 full turn--rechecked tps voltage and was at a .96

I didnt check the fuel pressure----butttt-when i got the truck, the cap is missing off of the test valve??? Obviously someone was in there before i bought the truck??? If the thermostat change doesnt work this weekend i will have to start next checking fuel pressure---any suggestions on what to look for???could this possibly give me a high idle??? It is concerning me now because the cap is missing off this valve ???

 

[mailboxman]

fuel pressure regulator

Latest update--i did change the thermostat and ATC air temp sensor in the intake and once again to no avail--i have been out of town but as soon as i got back this idle thing is making me crazy again--I checked vacuum at my fpr again and it is at a solid 20---i blocked the vacuum line and left it disconnected--the only difference I notice is the idle seems to hang a little more between shifts--is this normal??? Has anyone disconnected this vacuum line and had any different response? I understand this fpr actually lowers the fuel pressure to the injectors at idle? and opens up to allow more gas during low vac situations like acceleration? There is definitely no gas in the vacuum line or leaking from the fpr which means the diaphragm is not broken?? I will try to stop at one of the auto part stores tomorrow and put a fuel pressure guage on.
If anyone has disconnected that vacuum line with different results, please let me know and I will buy the fuel pressure regulator--again, i dont mind too much replacing parts on this truck because it does have over 160,000 miles--

 

[shaggymane]

very interesting JD--I actually pulled all the vac lines off and blocked all the ports on the vac intakes and ran the power brake hose to the pcv inlet--idle was fine--i then connected each vacuum line up one at a time--idle remained good--then I blocked the pcv line with a spark plug and connected the vacuum brake booster--still fine--i then pulled the spark plug out and reconnected the pcv valve and the idle climbed right back up???

I'll admit I don't know how a PCV works other than they positively ventilate the crankcase. If the little ball rolls around when you shake it, it's good right?
Anyway, seems like you'd be right close to the core problem there.
When you were checking for vac leaks did you spray ether around the grommet that goes into the valve cover? Sure seems like there's something going on below that pcv hose.

What happens if you connect the pcv to the hose with out the pcv valve being in the gormmet?

I'm watching this thread because I'm another one with the same problem. Mine was doing 1800 then down to 1200. Found a temp fix for mine but I'd still like to know the cause and cure.

 

[mailboxman]

What was your temporary fix shaggy mane??? I tried finding a loaner fuel pressure guage today at all the auto parts stores and nobody has one---i did get pep boys to bring me out a fpr and i tried sucking on the vacuum port and it acted just loke mine--i thought maybe i would be able to hear the diaphragm moving but sucking the port doesnt do anything--i will post my results to help you out if i ever figure it out--sounds like we got the same prob.

 

[shaggymane]

take the IAC gasket off from upper intake. See the two holes for the ports? They're about 1/2 or 5/8ths inch squares. Make a new gasket but don't put those large port holes in. Just use something like a 10penny nail to poke a holes in the new gasket, in lieu of the port holes. You know, one for each port. Hope that's clear

Only allowing a limited amount of air keeps the idle down. If the idle is to low make the holes a bit larger.
It's worked so far, even with using the peripherals......heater on hi, ps used.

It idles around 800 sometimes a bit lower when first started in really cold whether

I can't answer any of your questions about the FPR affecting idle this much. But can't see how it would
My gas mileage dropped. ran rough & rich. FPR was at 90psi. Replaced. Education stopped there.
The new FPR didn't change the idle problem.

 

[jd4242]

take the IAC gasket off from upper intake. See the two holes for the ports? They're about 1/2 or 5/8ths inch squares. Make a new gasket but don't put those large port holes in. Just use something like a 10penny nail to poke a holes in the new gasket, in lieu of the port holes. You know, one for each port. Hope that's clear

Only allowing a limited amount of air keeps the idle down. If the idle is to low make the holes a bit larger.
It's worked so far, even with using the peripherals......heater on hi, ps used.

It idles around 800 sometimes a bit lower when first started in really cold whether

I can't answer any of your questions about the FPR affecting idle this much. But can't see how it would
My gas mileage dropped. ran rough & rich. FPR was at 90psi. Replaced. Education stopped there.
The new FPR didn't change the idle problem.

90psi?is it still that high or was that before you replaced the fpr?shouldnt it be around 40psi,maybe something that high damaged your injectors?

 

[shaggymane]

90psi?is it still that high or was that before you replaced the fpr?shouldnt it be around 40psi,maybe something that high damaged your injectors?

I chop words to save on typing. "Replaced." For me that's a sentence.

30 to 40 psi is the range, IIRC. 35psi is the target

Oh yeah, FPR has been replaced. Wasn't gonna tolerate the 8 mpg very long. Runs fine after that except for that hi idle thing.
Gets 20 to 23 on the hiway. Idles real smooth with the gasket mod.

When i get more time I'm gonna experiment with my pcv area, see if there's a problem like mailboxman is having. I don't suspect any vac leaks on mine because if I pull any hose of the vac tree there's a noticeable change in the smoothness of it's idle. Haven't put the vac gauge on the nav yet.
Had it hooked up to a 2.3 mustang just to see. Pulled it over to 50. 50 what I don't know, the dial's cloudy

 

[2000StreetRod]

perform procedures

I suggest that you perform the TPS Test Procedure
followed by the Idle adjustment procedure

 

[jd4242]

I suggest that you perform the TPS Test Procedure
followed by the Idle adjustment procedure

I see that its for the sohc,will it work on the ohv? Also thats the first I've heard about the headlight trick,is that only for the sohc?

 

[2000StreetRod]

All engines

The photos are of my SOHC but the procedures should work for the OHV and V8.