Low RPMs @ Startup

[enjr44]

2002, 4.0L, Automatic, 2wd, cold start issue. Starting to drive me nuts.

At cold start after sitting over night or longer, about half the time the start up RPMs are about 1100 rather than the normal 14/1500. From then on after sitting for a couple of hours it always goes to the 1500 rpms and then drops to 1100 rpm after about 10/15 seconds. Only does it after sitting over night. When warm, it starts at the normal 1100 rpm and drops as the sensors come online (750/800 rpm).

It starts just fine, always on the first stroke, just like it has for the last 12 years. Idles fine, runs fine. No misfire, no hesitation, nothing but the low idle rpm at start after sitting over night. Kind of like the computer is not running the closed loop start up routine.

It only does this about half the time and it is starting to really bug me; waiting for the time it does something really weird. Like not start at all.

Any ideas???? Thanks, Ed

 

[RonD]

Couple of things could be happening.

The IAC(idle air control) valve could be sticking, this would normally set a code, CEL wouldn't necessarily come on for this code since it is idling high enough to not stall and is responding to computer control some of the time.

ECT(engine coolant temperature) sensor could be going bad, the ECT is only used by the computer, it is a 2 wire Sensor(dashboard temp gauge uses a 1 wire Sender).
The ECT sensor is sent 5 volts from the computer, the return voltage tells the computer the engine temp, the ECT is a resistance sensor, as it warms up resistance goes up so return voltage goes down.
At 60degF return voltage should be about 3.5volts
At 100degF return voltage should be about 2.5volts
At 200degF(operating temp) return voltage should be under 1 volt, .7volts

If ECT sensor is not going to the lower resistance when cold, so it is sending computer say 2.5volts, then computer thinks engine is just cooled off a bit after driving or it is hot outside, lol, but it won't set the higher idle just the 1,100rpms until engine warms up fully, .7volts
This would not set a code since ECT is working as far as the computer knows, voltage is changing as it should, it just thinks you moved to a hotter climate
This could set a code if IAT(intake air temp) sensor and ECT sensor were way apart, but probably no CEL for this either.

IMO, it would be worth getting the codes from the computer and see if anything points to the problem, most auto parts store read the codes for free, write them down, the code can be specific or general, and having the exact code is always required, the "I think it was Pxxx" will have you chasing your tail.

 

[Andy D]

Ron, would all that info pertain to a 94 4.0 ? That warm up scenario exactly describes the Rat's cold start. So 1500 rpm is normal? Great, so I wont fix that.

 

[RonD]

Ron, would all that info pertain to a 94 4.0 ? That warm up scenario exactly describes the Rat's cold start. So 1500 rpm is normal? Great, so I wont fix that.

Yes, pretty much all fuel injected engines use an IAC Valve for idle control, and have the same startup routine, not just Ford.
The computer opens the IAC Valve all the way at startup, injectors are set for rich mix, so 1,500+ RPMs is normal, then a drop in RPMs in a few seconds to match engine temperature, around 1,100 cold and 750 warmed up

 

[enjr44]

Ron, Thanks for your response. In the scheme of things this isn’t the end of the world; but, it is really bugging me.

One more bit of information, yesterday after sitting for about 20 hours it started at the 1100 rpm. After 5/10 seconds I shut it down without touching anything and restarted it. It immediately went to 1500 rpm and went through the normal start up routine. Does that tell you anything??

I am going to try turning the key from off to run three times before starting trick a few times to see if that changes anything. Also, I will change the IAC and see if that does it. I have a Napa one I bought about a year ago that didn’t fix anything (so I put the original back).

That was about a year ago and also for an idle problem. That turned out to be a dirty throttle body or MAF, I guess (was told). That was more or less the same problem, except that the idle would also drop to 500 rpm when shifting from drive or reverse to neutral or park and occasionally actually die. When I couldn’t figure it out, I took it to a local speed shop (with a great rep) and they put a couple of hours into it and said they couldn’t find anything wrong, so they cleaned all the usual stuff and it has been fine for the last year and now back to this (not actually the same; but, close).

Thanks again, Ed

 

[RonD]

If you have a 2nd IAC valve plug it in and see if it is opening all the way when key is turned on.

IAC valves use a Stepper Motor, a step motor has a digital like component, the computer sends the IAC valve a pulsed voltage "message" which causes it to rotate +/- XXX number of degrees which opens or closes the valve, the computer has the pulse commands in memory but has the ability to "learn" which pulses will give which RPM changes when IAC Valves are changed.
IAC Valve stepper motors usually have around 125 "steps" the computer can chose from, so it is very precise.

I would inspect the IAC Valve wires as much as you can, the drop to 500RPMs means IAC Valve was closed all the way.
Computer got the message you shifted out of gear, or engine would have stalled in gear at 500RPM idle, so either computer never sent "message" to opened IAC Valve or IAC Valve was stuck closed or IAC Valve never got the "message" to open.

 

[enjr44]

Thanks again, Ron. The drop to 500 rpm has not happened since the TB/MAF/etc. was cleaned last year. The low idle at startup began to randomly happen about a month ago.

OK, so I plugged in the new IAC I had on the shelf and it opens all the way. But, it only stays open for about 3 seconds. And then it closes (wasn’t expecting that). So, I removed the old IAC and it acts the same. Opens for about three seconds and then closes. Except there is one difference, the old one does a double bounce. Opens, snaps shut and then opens again for 2/3 seconds before closing again. Cycle the key back to off and repeat, get same response each time (opens, bounce shut, open again, shut after 3 seconds). I guess the three second thing is like the fuel pump. On for three seconds, then off if no crank rotation.

With WOT and going to start the new one goes to full open each time. The old one opens; but, I’m not sure it went to full open each time. So I installed the new one. On first start it idled a 1500 rpm like it should. Will keep my fingers crossed. Since the truck always starts correctly for the rest of the day once it has been warmed, I won’t be able to tell anything until it has set over night again.

Will let you know and thanks, Ed

 

[RonD]

Yes, that is normal start up for the IAC, the computer tests it(opens all the way) then closes, then waits for CKP(crank position) sensor to start sending a pulse(crankshaft turning via starter motor), computer then starts spark and fuel injectors and opens IAC valve.
Often IAC Valve opens full, 1,500+ RPMs at startup, regardless of engine temp(ECT sensor), then closes to match engine temp and it's target RPM.
But this is software dependent so not written in stone.

And there is also the A/C to be considered, in Defrost mode the A/C compressor will come on to dry the air inside, which changes target idle, so if defrost was selected last time vehicle was driven the idle may stay higher than expected, which is normal if A/C is coming on.