Cold Start Low Idle/Runs Rough

[Guggenheim99]

02 4.0 having an issue with a unusually low idle on cold start and can feel it running slightly rough. For years since I bought the truck, it would have an intermittent issue where I would start it cold and would have a pretty low RPM, like 1k or less. Sometimes tapping the gas real quick would kick it up to what I would call normal cold start RPM (1,500rpm) Sometimes it would start and be normal. I just lived with it. But last week I changed the fuel pump/sending unit for a unrelated issue (dead spot in the pump) (NOS OEM) and the condition has gotten worse. When I cold start it, now it will go to maybe 500rpm and can feel the engine running slightly rough. Tapping the gas like I mentioned above gets it to maybe 1k RPM sometimes, but other times makes no difference. Any ideas? Tomorrow I'm gonna look at some live data when I do a cold start to see if I see any anomalies. Truck starts normally/as designed when engine is warmed up. Thanks

 

[franklin2]

When is the last time you cleaned the throttle body blade and the idle air control valve? They get all gummed up and need to be cleaned after several years of use.

 

[Guggenheim99]

When is the last time you cleaned the throttle body blade and the idle air control valve? They get all gummed up and need to be cleaned after several years of use.

I've cleaned the IAC I think 2 or 3 times over the last 7 years I've had it and the throttle body once as general maintenance and it never made a difference. I was fine with it since the issue wasn't as pronounced as it is now

 

[Guggenheim99]

So I caught it in the act on Monday with Forscan running. I unfortunately didn't have all the PIDS I should have pulled up but what I did find is on cold start when the condition occurs, the PCM is commanding the IAC valve at a lower % duty cycle then it should be (67%) and desired idle speed was 1440RPM, actual was I think less then 1,000RPM. ECT sensor reading normal ambient temperature (after 7 hours sitting outside) Today the condition did not occur and this time desired and actual idle speeds matched, duty cycle for IAC was around 80% I think. Also added in TPS voltage and that looked good (varied correctly through entire range and returned to the specified lowest voltage of like .18v as per the PID reading) Who knows if it is actually good or not since it didn't act up today. I plan to find more PCM inputs and keep trying to get it in the act and see if any readings look abnormal. At this time the IAC valve appears to be working correctly since it's following the duty cycle % correctly. Do you guys know the inputs the PCM uses to control the IAC valve? I would think a combination of MAF, ECT, IAT, O2 sensors, TPS. Any I am missing?

 

[franklin2]

I would think a important one would the where it gets its rpm information. I guess that would be the crank sensor. I suppose you would need some other way to verify the actual rpm of the engine. The tach in the cluster probably uses the same signal as the engine computer does for rpm.

 

[Guggenheim99]

I have data logged over the last few days and I am starting to think it's not an input issue. All the data looks ok. I unplugged the MAF just to see what happens to try and put the PCM into a default strategy and didn't make much of a difference. Once I had it running I shut it back off and immediately restarted and it ran horrible. Running very rough. I shut it off and key'd it on and off a few times and restarted and it ran even worse. But will clear up after about 5 seconds. Beginning to think it may be fuel delivery related but I don't know for sure. Like I said it's always had a intermittent low cold start idle that I could resolve with tapping the gas but after I put in the fuel pump/sending unit assy in it has gotten worse. I can still resolve it with tapping the gas but it takes 2 or 3 tries. Just don't understand why it starts up no issues once the engine starts to get some heat in it. Probably gonna check fuel pressure and see if I see anything abnormal. Where is the test port on a 4.0 SOHC? I looked in shop manual and didn't see it and don't see it in the engine bay. May have missed it. Any ideas are greatly appreciated.

 

[franklin2]

I tried to look it up. All it said was all engines have a test port and it's located on the fuel manifold on the top of the engine somewhere.

You can listen to my little problem I just had with my older Bronco II and see if it may give you a idea.

I retro fitted a fuel injected 2.9 in my 84 BII that originally was a 2.8 carb. So I had to swap all the fuel injection system over. Everything went smooth, I drove it all year from last spring to a few weeks ago. As I was using it, I noticed, especially on a cold morning start, that it cranked excessively before starting. Then I caught it a few times, it was actually starting when I let off the keyswitch. I had another column with a new ignition switch, and my column had a worn out lower bushing, so I changed the whole column out. Didn't fix it.

As it got colder and colder, the problem would get worse. I found on a morning start, I had to hold down the pedal partially, and keep cranking and then letting up. If I cranked too long it would flood. It got to the point it would not hardly start at all so I did some serious wiring and diagrams inspection, and a bunch of testing.

Come to find out, there is a wire that comes from the ignition switch on the BII that I tied to the red/lightgreen wire in the EECIV harness. This red/lightgreen powers the EEC relay, the ignition coil and the TFI ignition. I used the white/lightblue from the 84 harness to power this circuit from the keyswitch. But apparently it's not large enough. When it's cranking, there is too much voltage drop in this circuit, and I am getting a weak spark. I found that if I ran a jumper wire to this circuit straight from the battery,, it starts right up, and it was about 20 degrees outside today.

Which leads me to your problem. I wonder if you can pick a critical spot in your EEC harness that supplies voltage, and monitor this during a first start up and then after you tap the throttle. I wonder if the voltage is jumping up significantly on one of your circuits when you tap the throttle. Of course if the voltage then stays up, then the engine is going to run better, which will make the voltage stay up because the alternator is spinning faster.

 

[Guggenheim99]

So, today I went out and cleaned the MAF and IAC just for the hell of it cuz I had spray for both laying around. No change. Once it stops stumbling, if I shut it down, key it back on and start it up again, it stumbles again, worse then the last time. Keep repeating this and it gets worse and worse each time. Can smell a rich fuel mixture exhaust smell after a few times doing this. No black smoke seen, just steam since it is cold out. Holding the throttle open slightly during the condition to rule out an IAC issue does not improve condition at all. Like I said once I get some heat in the engine it starts as designed. I was able to find a factory 2002 Powertrain Control/Emissions Diagnosis manual (which the factory shop manual mentions quite frequently) for sale on Ebay and have that on order. Maybe I can find more info on theory and operation in there. I did find the fuel pressure test port. At least in the Ranger 4.0 SOHC it's on the back of the passenger side fuel rail. I didn't test it because it was just way too cold and windy out today lol. I'll update if I find anything. As always any ideas are welcome.

 

[franklin2]

You have to remember also that a lot of the devices in the computer system do not have much influence when the engine is cold. The engine runs on the set tables in the memory of the computer till the oxygen sensor gets warm enough. Of course it looks at a few, it would not run without the crank sensor and the coolant temp sensor. I suppose it does look at the MAF but as you found out it will run without it.

It sure does sound like it's miss-firing and not completely burning all the fuel when it's cold. I think it's a old school ignition problem.

 

[Guggenheim99]

So update finally got around to putting a fuel pressure gauge on this. Key on, pressure almost 70psi, then drops to 65psi after fuel pump shuts off. As per PC/ED manual, it states 5psi drop after 1 minute is spec. After 1 minute, it dropped 4psi, but kept dropping after that. Think within 10 mins, it got to around 50psi. Went to next step to check fuel pulse dampener for fuel in the vacuum line. Well that is buried at the back of the fuel rail under the coil and a ton of wires and hoses. I can barely even feel it. So I checked where the hose goes into manifold and no fuel present. It then states to check injector flow rate/leakage. I was going to pull the intake manifold to check the runners for fuel. But I noticed the gauge stopped dropping and the condition appears to be not happening anymore (either I fired up the truck too many times and it warmed up a little or the heated garage I am in is affecting it) But I noticed alot of moisture around the passenger rear-most injector. I took a thin screwdriver and probed around down there and can smell fuel when I smell the screwdriver. And the body of the injector seems extremely dirty and wet. I will attach 2 pictures. Is it possible the rail to injector seal is leaking down the body of the injector, past the o-ring where it seals against the head, and down into the runner? Maybe an injector issue?

 

[Guggenheim99]

So that fluid was just from me hooking up the gauge and not an actual leak. But I removed the intake today and found a little bit of fuel pooled up on 4 of the 6 intake valves (the other 2 were opened) Thought it was odd but I cleaned out all the runners, put the pressure gauge on, key'd it on a few times and pushed it outside. 4 hours later come back out and it had dropped from 60psi to 10psi. I checked the runners and found none of them have fuel. Feel if I had leaking injector it would be be steadily dripping and the runner would be full. I went back to the pinpoint test for fuel pressure leak and it points to fuel pump module. The rough running did start after I put an OEM pump assembly in for an intermittent fuel pump issue. Now I've got multiple issues; rough running ONLY when it's cold soaked, fuel pressure dropping, smells like it's absolutely pouring fuel on startup, and the low idle. I did put a Ford IAC on but I have been unable to say if it has helped with it running rough. Pretty lost. Could fuel pressure be bleeding back to the tank? But I don't have an extended crank, and how could that cause it to run rough and smell extremely rich on startup? Just a reminder none of these concerns occur when the truck runs for a couple minutes. It will snap out of it. Any help is very much appreciated

 

[franklin2]

Yes, sit down and go over what you did, and when the trouble started, and how many troubles you had in the beginning, and how many do you have now. It's not unusual to install faulty new parts. It's also not unusual to foul something else up when installing a new part. They cram so many things together and you have to tear half the truck apart to replace something it's easy to break something else, pinch a hose, forget to hook something up, etc.

 

[Guggenheim99]

So another update on this tough diag. I put a Delphi fuel pump assy in since originally this whole thing began after I replaced it previously but that didn't fix anything. Must be a coincidence. I then tried blocking off the purge valve nipple on the intake manifold because I thought maybe the valve could be leaking fuel vapors into the manifold causing a hard start but that didn't fix anything either. So I hooked it back up. I had tried to look at mode 6 misfire data with Forscan but they give a % value and they all seemed low so that confused me. I bought a Ford NGS to look at misfire data that wasn't in % values; but of course it will not link to the truck. Have continuity from the power, ground, programming, and ISO bus pins at the DLC end of the cable to the tool end but have no continuity on the SCP + - lines. So now waiting on the guy who sold it to me. Anyway, I unplugged the IAC and put a thin metal stock between the throttle body and the stop screw and the truck would still continue to act up even with more air being introduced into the engine. When removed, the engine died so I have no vacuum leaks at least. So it seems I got air and fuel. Highly doubt it's a compression issue. Don't think I have coolant leaking into a cylinder because the exhaust does not smell like coolant, just extremely rich fuel. I'm down to spark now. Possible a weak coil? I wish the NGS was working because I need to know if this is just a single cylinder or multiple cylinder misfire. ECT is fine, MAF seems good, IAC is working. Not really any other inputs during open loop. Whatever issue it is, it snaps out of it when I snap the throttle. Any input is appreciated as usual

 

[franklin2]

Does the 2002 model have a fuel pressure regulator on the injection rail with a vacuum line? That is a common spot for extra fuel to get into the engine on the older trucks when the regulator leaks.

 

[Guggenheim99]

Does the 2002 model have a fuel pressure regulator on the injection rail with a vacuum line? That is a common spot for extra fuel to get into the engine on the older trucks when the regulator leaks.

No, regulator is on the sending unit assy inside the tank. There is a fuel pulse dampener on the fuel rail. I did verify no fuel was getting into the vacuum line for it.