Funny Idle

[Brandon_Kelsey]

Hey, so in my 1992 4.0L Ranger with 103k miles. It idles very weird sometimes. Mostly when I start up after the engine is hot. Like if I cut it off after driving for a bit, then go in to the store for 10 minutes or so. Has no problem starting but it only reaches about 1500 RPM when it usually reaches about 2500. But then it drops to about 500 RPM after its ignited. I have to rev it up to keep it running. If i put it in reverse it sounds like its going to stall on me. I have done the plugs and wires, a transmission flush at about 99k, ive cleaned the IAC valve and put a new gasket on. I unplugged the TPS, and MAF for a little while and plugged them back in. Nothing has worked yet... I will be doing the fuel filter and a radiator flush very soon. I took it to a shop and they plugged it in to the old computer, and he checked all my vacuum hoses. Nothing came up.
Any ideas what else could be causing this or any tests I could do?
Could it be the fuel filter?
Anything with the EGR system or intake?
Anything willl help! Thanks guys!

 

[downer]

it would be worth it to temporarily block the egr outlet. the valve would likely be sticking open if it is the problem here.

cut a small piece of metal, loosen the egr valve bolts and sandwich it between the egr valve gasket and intake tube. it will eventually trigger a cel, and should only be used for troubleshooting.

 

[Brandon_Kelsey]

so with that test. what does triggering the cel do? and how do i determine if the egr valve is the cause or not?

 

[RonD]

EGR is not a bad guess.
But if it starts and runs OK cold I would just remove vacuum hose from EGR Valve and plug it, the hose, and then drive it as normal, yes CEL will come on eventually, you can reset it later.
If hot restart symptoms do not reoccur then you found the problem.

EGR system has 3 parts, 4 if you count the computer.
1. EGR modulator/solenoid, this is an electric valve that the computer controls to apply vacuum to the EGR Valve, it will have electrical connector and 2 vacuum hoses, 1 to EGR Valve and 1 to intake manifold(vacuum source).

2. EVP or DPFE sensor, this tells the computer if EGR Valve is opening and how much.

3. EGR valve, vacuum operated valve on intake manifold that allows exhaust gas into intake when engine is under load, the cylinders/pistons in an engine under load will heat up, this increases NOx emissions and can also cause pinging/knocking(pre-ignition).
Mixing exhaust gases with air/fuel mix cools the cylinders/pistons, yes, lol, go figure.

So it may not be a bad EGR valve, if it is the EGR system causing the problem.
It could be EGR solenoid leaking vacuum to EGR Valve or EVP sensor issue


But one puzzling thing.
The computer sets the idle at 750-800 for automatic when engine is warm, using the IAC Valve.
So if EGR valve was opening and fouling up the air/fuel mix, and idle was dropping to 500 because of that, then computer would open up IAC valve more to get RPMs back up to 750, it wouldn't stay at 500.
Computer constantly monitor RPMs and it opens and closes IAC Valve to maintain that RPM range.
Computer gets RPM from CKP(crank position) sensor and it is pretty accurate.

So I would recheck the IAC valve.
The IAC valve uses a stepper motor, this is a motor that has a "digital" rotation map, the computer sends it a "digital pulse" on the power line, and motor turns to a preset location, there are over 100 preset "steps" on an IAC Valve motor.
Computer opens IAC Valve all the way on start up, the 2,000+ RPMs you see, then closes it to cold or warn idle levels depending on ECT sensor(engine coolant temp), but never below 700 RPMs.

So my guess, if it is not the EGR system, is that the stepper motor could be failing when it gets warm
Computer is sending it the correct pulses but it is not moving as planned and computer can not get it to respond to raise idle.

 

[Brandon_Kelsey]

So, having my EGR unhooked from the vacuum source is not harmful, correct? ( i dont mess with EGR's much so i just want to take some precautions) And also, your saying if the funny ilde with a hot start doesnt happen, then my EGR is the issue?

 

[RonD]

Yes, you can run with EGR valve vacuum hose disconnected, plug the hose with a screw or bolt as it will be a vacuum leak when engine is under load.
I will not hurt engine, CEL light will come on but usually take a few minutes of driving.

Yes, faulty EGR System(not just valve, see post above) can cause idle problems if EGR valve is being opened at the wrong time.

 

[Brandon_Kelsey]

Ok, i will do the test tomorrow and post after its done. Thanks for the help.

 

[Brandon_Kelsey]

Okay so today i spent an hour just looking for my EGR. Does a 1992 Ranger with a 4.0L OHV even have an EGR? SOme sites say no and some sites say yes. Can anyone verify for me?

 

[SenorNoob]

I'd say it's fairly likely that it doesn't. Just look for the metal pipe running from the exhaust to the intake to determine if it does or not.