Solved Coasting to a stall (SOLVED)

[Coasting2aStall]

Ok, this is 1994 Ranger with 4.0L OHV and manual transmission. No codes. OBD 1 so any codes probably pretty vague if they did exist. Think of this as a 1993 Explorer drivetrain.

It starts easily cold, runs fine, no stalling. Cold idle around 800rpm. No roughness. After engine warms up and I coast to a stop, it stalls. Quick and clean stall, no hesitation, like if I had turned engine off. You can see it all of a sudden drop 600rpm on tach to zero. Restarts easily and will idle at that point around 600rpm. No hesitation accelerating. May or may not rinse and repeat next time I come to a stop. IT WONT DO THIS IF ENGINE STILL COLD!

Previous owner replaced some stuff including, I suspect, MAF sensor, as its clean, pretty and shiny. I pretty much replaced the rest including new fuel pump and new fuel pressure regulator, oxygen sensors, coolant temp sensor, and throttle position sensor. The IAC in particular, no less than THREE times. Ranger of this era didnt have an EGR so that isnt a consideration.

 

[Roadrunner777]

This is classic MAF sensor failure, and I wonder perhaps if the previous owner cleaned it trying to solve the problem. I have had this issue twice, fixed it twice by replacing the MAF. There are some MAF tests you can run measuring voltages on the MAF leads. I don't have a link, but I seem to recall it googled easily.

 

[Coasting2aStall]

Ok, little update. Just out messing with it. Pull the connector to the MAF and it still stalls when engine warmed up. Pull connection to the IAC and it still stalls. Pull the connection to the TPS and idles little rough and pours out dark smoke from tailpipe. But it doesnt stall no matter what I do.

Oh yes, it is now happening at warm idle all time, no need to be coming to stop out on road.

Guessing that its obviously running very lean (and stinkin rich with TPS unplugged) but shouldnt it be running limp home mode with MAF unplugged and thus MAF not a factor? If its the MAF, why is it stalling with MAF unplugged?

Oh and if I wanted to replace the MAF without spending a fortune are there any brands on low end with good feedback? Rockauto.com has a plethora of them to choose from. From previous experience, would guess either Hitachi or Motorcraft be best bet though more expensive. Sometimes the cheapest man pays the most. And sensors many times are one of those times.

 

[Coasting2aStall]

Further update. Testing MAF, its 10.5V for two wires on one side and .84V for two wires on other side. (there are four wires total) This with plug connected and engine running. Getting it tested before it stalled was priceless.... Anyway this was in range specified by the article. The TPS also tested ok. I tested it just for giggles cause I found article how to do it too.

 

[Roadrunner777]

Running lean.... vacuum leak? I'm out of experienced ideas if the MAF is ok. I myself might hook up a fuel pressure and vacuum gauge and see what they tell me. I have rigged them up more than once so I can see them while I am driving, just to test. Good luck... I don't visit here frequently, I hope someone who posts more regularly chimes in.

 

[Coasting2aStall]

Running lean.... vacuum leak? I'm out of experienced ideas if the MAF is ok. I myself might hook up a fuel pressure and vacuum gauge and see what they tell me. I have rigged them up more than once so I can see them while I am driving, just to test. Good luck... I don't visit here frequently, I hope someone who posts more regularly chimes in.

Thanks. Since fuel pump and pressure regulator are new, very likely they are ok. The pressure regulator got replaced with cause, diaphram leaked gas which was sucked into manifold through vacuum line. I replaced the pump just for good measure as I had no clue if or when it had ever been replaced and if pressure regulator leaked...

And since this seems to be worsening problem, and only when engine warm, vacuum leak sounds very likely. However that covers lot territory on modern engine. Have to be a relatively big leak too, more than what the computer can compensate for.

 

[Rhett]

Have you looked at the ECT sensor? It's signal changes depending on whether or not the engine is warmed up, and a bad one will cause all sorts of issues with idle and stalling. O2 sensor inputs change with a warm engine also.

Aside from a large vacuum leak, other possibilities (some of which you seem to have ruled out) are: fuel pump, fuel pressure regulator, PCM issue, O2 sensors, transmission vacuum modulator, IAC, TPS, EGR valve (if equipped), PCV valve, engine coolant temp (ECT) sensor.

Finally...and this is a problem I had....check the throttle set/throttle adjustment screw on the passenger side of the throttle body. This screw can fall out, or move over time. When your engine warms up the ECT sends different voltage to the PCM, and it commands a lower idle RPM on a hot engine...if your set screw is out of adjustment, it could mean that your idle is fine for cold engine, but too low for a hot engine -- due to the set screw being too far out, or missing (mine was missing).

 

[Coasting2aStall]

The ECT is the coolant temp sensor for ECM, right? On front of manifold under the throttle body? Yea it got replaced. Like I say just about everything replaced by either previous owner or me in last couple years. Though any sensor one cant easily test, could even be bad new out of box. Though whatever problem is, it changed just recently from occasional stall coasting to a stop, to simply not staying running at idle more than minute once engine is warmed up.

Oh the throttle adjust screw. I had looked at it in past. It is in there tight and the head is round and has no slot for screwdriver. I am taking it factory didnt want it owner adjusted..... Though its obviously threaded so I suppose one could turn it with small pair vise grips. Or whatever specialty tool Ford has for it. I will check it but as tight as it was, if it somehow changed on its own, I would be amazed.

Hey do you know if all vacuum lines on this engine originate at that spider fitting on upper manifold on drivers side back by firewall? I am wondering about plugging everything there temporarily to see if that has any effect. I suppose could be leak in some hidden bit vacuum line I missed. A leak in a vacuum line makes more sense changing that rapidly than a manifold flange leak.

 

[Rhett]

The ECT is the coolant temp sensor for ECM, right?

Yep that's the one.

Oh the throttle adjust screw. I had looked at it in past. It is in there tight and the head is round and has no slot for screwdriver. I am taking it factory didnt want it owner adjusted..... Though its obviously threaded so I suppose one could turn it with small pair vise grips. Or whatever specialty tool Ford has for it. I will check it but as tight as it was, if it somehow changed on its own, I would be amazed.

Well, all I can say is, mine fell out completely, and, being threaded, despite being in there tight they can thread out over time. That screw bangs against the TB constantly, every time you touch the gas.

Since it's easy and cheap, I would crank the screw clockwise, at least 10-15 turns. It should raise your idle, hopefully enough that it will not die when you're coasting to a stop.

If that doesn't do it, report back because there are other possibilities...as I mentioned in my previous post (like the vacuum modulator, which controls the 1-2 / 2-1 shift).

 

[Coasting2aStall]

Been playing with it. Tried blocking all the vacuum ports including one for pcv. No difference. Then I loosened the IAC and slipped strip cardboard to block ports and tightened it down. That made a difference. Idle was little lower than usual, but rock steady and lasted for long time.... until it just died like I had turned off the ignition. Remember it no longer requires coasting to a stop to stall. It does it now with truck parked in my yard and transmission in neutral.

Oh yea, the throttle screw is in there tight as can be. I was able to move it with small pair vise grips but it took some effort. But unless I want to set idle at 2000rpm, dont think higher idle speed is going to keep it from stalling. Its not shuddering or acting like its starving for fuel or anything, just like I turned the key to off position all of a sudden. Starts right up. But once its that hot, it will stall quickly again. Very annoying.

And the vacuum modulator is not a factor. Like I said this is a manual transmission.

There is either a vacuum leak I am not finding or some other reason for it to run lean at idle. I was looking for leak in tube from MAF to throttle body. Unmetered air there might cause lean condition at idle. But I didnt find a leak.

 

[Coasting2aStall]

Ok, may found an answer and its a doosie. Very counter intuitive. Rhett suggested turning in the throttle screw so it would be high enough rpm it wouldnt stall. I didnt think it worked like that as TPS doesnt act independently of air flow, etc. Computer ties everything together to figure out how much fuel to send. But got me wondering how adjusting TPS would affect things. There is no adjustment, but you can remove screws and turn it a bit and hold it. Ok not too helpful, still stalled. But a tiny bit of a turn either way at idle made quite amazing difference.

Ok, well what happens if I turn the throttle screw out so less air flowing through the butterfly. So turned it several turns out. At first rpm went down a bit, then up and normal around 700! Engine got lot quieter. Temperature gauge went into middle of Normal. It didnt stall out!!!!! and I let it run good long time. Ok, maybe a fluke, tomorrow I drive it out on road a bit and see if it stalls as I coast to a stop or anything.

My guess is that some previous owner screwed the throttle screw in to get higher rpms and instead got leaner mixture because of the way the computer combines the various sensor readings. Still only a theory, see what happens tomorrow. But not stalling and quieter engine is very promising.

 

[2stroke]

It's certainly not impossible. There is a way to make the TPS adjustable, you simply mill out the holes a bit. It's not hard to adjust properly. With a volt meter, the voltage with the throttle closed should be .96-.98 VDC. I like to start by warming up the engine, and unplugging the IAC.. If it stalls, turn in the throttle plate stop screw and try again. Once it's running, adjust so it idles about 500-525 rpm with the IAC unplugged. Once the idle is set, adjust the TPS so the voltage reads .96-.98 VDC. Then plug the IAC back in, and you now have a properly adjusted throttle, and TPS.

 

[Rhett]

Like I said this is a manual transmission.

You said that? I must have missed it. That changes everything.

Don't you M5OD guys have a neutral safety switch? Maybe it is not behaving.
How about the slave cylinder...is it functioning properly?
Is the clutch not fully disengaging?

You really need to discuss this with a manual tranny person, and I am not that guy, but the above are good places to start.

 

[Coasting2aStall]

I am just saying there is no vacuum modulator on a manual transmission. So a problem with a vacuum modulator is not something I have to consider. Nothing on a manual transmission would have anything to do with hot idle stall. Rainy day so I still havent taken it out for test drive. Pretty sure stalling problem solved with the turn of a screw, but proof is in the pudding and wont know for absolute sure until decent test drive.

 

[Rhett]

I am just saying there is no vacuum modulator on a manual transmission.

I know that. You just didn't make it clear that you had a manual, or I missed that. Hence, I would have never brought up the vac mod.

Since you have determined that a hot idle stall has nothing to do with a malfunctioning manual transmission, I'll leave it to you....adieu

 

[Coasting2aStall]

I know that. You just didn't make it clear that you had a manual, or I missed that. Hence, I would have never brought up the vac mod.

Since you have determined that a hot idle stall has nothing to do with a malfunctioning manual transmission, I'll leave it to you....adieu

Uh, first sentence of my original post! "Ok, this is 1994 Ranger with 4.0L OHV and manual transmission". But adieu. And manual transmissions are ...gasp... controlled manually. Thus the name. They are not reliant on engine vacuum nor engine computer. At most you have a clutch interlock so starter wont engage unless clutch is pushed. No function once engine is running. Somebody long ago removed that from my Ranger. Oh and backup light switch.

 

[Coasting2aStall]

Ok, officially declaring this problem solved. I am just back from test drive. Not only didnt it stall, but it had wee bit more power. And thanks for suggestions. Especially Rhett as he was one suggesting changing the throttle stop in first place that got me thinking more about TPS and how the throttle stop affects the computer. Seriously, if not for that little brain nudge, I would be elbow deep in intake manifolds trying to replace bunch gaskets and seals. And I truly would be unhappy camper if I had replaced all that and still had the stall problem.

 

[Roadrunner777]

Well done, sir!