Idles like it's cammed

[Aaron Powell]

so my 94 b3000 5 speed with a 2004 Ford Taurus 3.0 intake manifold and throttle body idles like it's cammed and it has a hard time adjusting a/f ratio when i give it throttle (it chokes on to much air) unless I ease into and "pulse" the throttle pedal if you will when the MAF is plugged in. I've replaced it and warrantied it twice (oreilly blue streak MF0374N)(it is possible I keep getting bad ones. I've had this problem with other electronics before). If I un plug it, it idles fine, changes idle accordingly ie compressor turning on/off and demand from alternator. I've capped of all vacuum and very lightly sprayed brake clean looking for a vacuum leak as well as spraying brake clean around the entire engine bay like a bukkake. To no avail have i found a leak.

On another note, I had the same issue before the manifold swap. My next thing is to test fuel pressure. I was going to replace the IACV, but the fact that she runs fine w/o the MAF tells me the IACV isn't bad. What am I missing? No CEL unless the MAF is unplugged.

 

[RonD]

Could be the MAFs wiring, half way down here are some voltages to check: https://www.therangerstation.com/tech_library/CleanMAF.shtml

With MAF unplugged computer stays in open loop, but even in closed loop the computer ignores O2 sensors at idle

Vacuum leak will give you a higher idle, MAF plugged in or unplugged wouldn't matter

Check PCV valve, often over looked system, its a "planned" vacuum leak but ONLY when vacuum is lower, i.e. accelerating or higher RPMs
The weighted valve in the PCV Valve is sucked UP to close off the valve when engine is idling(high vacuum), then when vacuum in intake drops, so does the weight which opens up the PCV Valve to suck in Blow-by gases
So you could get a nice "cam" rhythm going with a sticky PCV Valve, lol
But yes, doesn't fit with MAF..................but FREE to check it out

 

[Aaron Powell]

Could be the MAFs wiring, half way down here are some voltages to check: https://www.therangerstation.com/tech_library/CleanMAF.shtml

With MAF unplugged computer stays in open loop, but even in closed loop the computer ignores O2 sensors at idle

Vacuum leak will give you a higher idle, MAF plugged in or unplugged wouldn't matter

Check PCV valve, often over looked system, its a "planned" vacuum leak but ONLY when vacuum is lower, i.e. accelerating or higher RPMs
The weighted valve in the PCV Valve is sucked UP to close off the valve when engine is idling(high vacuum), then when vacuum in intake drops, so does the weight which opens up the PCV Valve to suck in Blow-by gases
So you could get a nice "cam" rhythm going with a sticky PCV Valve, lol
But yes, doesn't fit with MAF..................but FREE to check it out

I think I'll replace the PCV then, wouldn't hurt anyways. I hear it "tapping" at idle. What you explained makes perfect sense. I'll also check voltages, thank you for that link.

 

[Aaron Powell]

So, I replaced the PCV and warrantied it the MAF again, and no fix to the issue. I checked for vacuum leaks, again, and found none. She runs fine w/o the MAF plugged in, she just runs on the rich side. When the MAF is plugged in, it's like cylinders die. She smells more rich and misses heavily until she dies.

 

[RonD]

Then computer issue most likely

 

[Aaron Powell]

Then computer issue most likely

I think I read somewhere that the ECT sensor for the ECM not the cluster gauge, can cause issues also if it's no good. The problem was a slow onset that happened all in about 2 weeks.

 

[RonD]

Sure but would set an ECT code if voltage was varying enough to cause described issue

ECT sends about 3v of 5volts back to computer when engine is cold, that sets Choke mode
Then as engine warms up the 3volts drops to below 1volt when engine coolant temp is 180-190deg
Computer has a timer that also runs so if ECT voltage doesn't start to drop within a few minutes of run time it will set a code
And if voltage is not stable, goes up and down, it will set a code

Its possible and ECT is not an expensive sensor to change

 

[Aaron Powell]

Sure but would set an ECT code if voltage was varying enough to cause described issue

ECT sends about 3v of 5volts back to computer when engine is cold, that sets Choke mode
Then as engine warms up the 3volts drops to below 1volt when engine coolant temp is 180-190deg
Computer has a timer that also runs so if ECT voltage doesn't start to drop within a few minutes of run time it will set a code
And if voltage is not stable, goes up and down, it will set a code

Its possible and ECT is not an expensive sensor to change

I replaced the ECT sensor and still nothing. I've replaced the MAF and I even did the PCM. Same exact issue. It was a slow onset issue. I removed all the harness grounds and sanded the mounting points on the chassis to bare metal and cleaned off the ground terminals in the harness and re attached them, still no change. In going to go through voltages again. Does the MAF on the 4.0 and 3.0 have the same voltages/tolerances?

 

[Aaron Powell]

I also get no CEL, when the truck is running very poorly and eventually the engine dies. Dumb question, will the air temp sensor that goes in the air box cause this if it's bad?

 

[RonD]

I wouldn't think so without setting a code
The IAT or ACT sensor doesn't vary that much, outside air temp and then running engine air temp, it's basically a fine tuning sensor, mostly used in Choke Mode, just after cold start, so computer knows how much Choke to apply in Cold Weather, after warm up it wouldn't vary too much and would set a code if it did, but couldn't put computer back into choke mode

 

[Aaron Powell]

I checked voltages again. Power, 13.1-7
Signal, .9 at shitty bouncing/missing idleand once I get her revved up, 3.1 volts. She just threw a light. I'm going to check codes really quick

 

[RonD]

You can test it without engine running, but key on, its air temp, so a few degrees less than outside temp because its "in the shade", lol

Use same scale and voltage as ECT sensor, if memory serves

 

[Aaron Powell]

Sorry, I should have specified, the voltages I posted were for the MAF.

 

[Aaron Powell]

All the codes I got were sporadic from me messing with things. I cleared the code and the did a KOER test and got 412 idle not controlled (too low) and 538 system did not receive goose test (not worried about that).
After she runs for a few minutes, she'll figure it out and idle better for a few minutes then go back to "loping" like she's cammed and misses and I have to gather the throttle in and out to get the rpms to climb and she still misses and runs super rich, rich enough to burn the powder coating off of the glass pack. When she smooths out and runs good, throttle response is great with no misses.

 

[Aaron Powell]

Another note, if I un plug the IACV weather she's running smooth or not, she dies. If I un plug the TPS while running smoothly, she shudders and then idles smooth. If she's running rough and i in plug the TPS, she shudders a little then back to running like shit.