02 sohc with random no-run condition - Ranger-Forums - The Ultimate Ford Ranger Resource


4.0L OHV & SOHC V6 Tech General discussion of 4.0L OHV and SOHC V6 Ford Ranger engines.

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Old 07-05-2014
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02 sohc with random no-run condition

This is my first post here so I'd like to say hello everyone. I haven't really done any of my own work on this truck until the past year or so. I can't afford to pay mechanics to work on it anymore and the past couple times I went to a shop all they did was rip me off and tell me they couldn't find the problem. This truck is now my only vehicle so I need to to be back in good condition so I can actually depend on it. So I appreciate any assistance you all can offer.

Truck currently has 102k miles

No modifications to original factory specs

The problem: I can only get 2-4 successful starts in one day. The truck doesn't have a problem maintaining an idle, but if I have to take more than a few trips in a day it will randomly not start. When this happens the engine will rotate but will not turn over. Occasionally I will pull these four fuses for 10 minutes: fuel pump, pcm memory, pcm diode and pcm maxifuse. Then the engine may start, or will start and then die after a second (throttling doesn't maintain the idle, it causes the engine to shut-off immediately).

My only solution when this happens is to abandon the vehicle for 8-24 hours and return, and usually it will start.

The last shop I took it to told me they found codes for a failed IAC. They were also a bunch of bumbling morons, but I'm not going to get into that now.

Fixes I have tried:
Replaced IAC motor and IAC gasket
Replaced EGR valve and gasket
Replaced PCV valve
Replaced alternator (the post had melted off the previous alternator which had only lasted a year)
Replaced drivebelt (sched maint)
Replaced spark plugs and spark plug wires (sched maint)
Cleaned throttle body
Cleaned MAF sensor
Replaced TPS (had no effect on incorrect throttle position reading)
Checked for vacuum leaks with carb cleaner, none found

The only problem I could find doing my own diagnostics with an engine scanner is that the throttle position when the engine is off or idling is 18.4%. I replaced the TPS and it shows the same reading, however after running the truck with the new TPS it settles at 19.2% now. I tried a volt test of the signal coming out of the new TPS and instead of smoothly moving between .1ish and 2.3ish V, it just sticks at .33. However, if it fails this test wouldn't the PCM not be able to receive a signal? I could be performing the test wrong. Watching the engine scanner the throttle position moves smoothly between the 19.2% and 93ish% both on the old and new TPS. There is a good 11.5-12.5v running to the unit so that isn't an issue.

I'm about to replace the fuel filter soon now that I've learned how to make the clamshell tool out of a sharpie cap (lol).

I appreciate any help you guys can offer! A family friend of mine who has done a lot of work on Ford 3.0 and 4.0's told me it could be the fuel pump, but that is a job I'm on the fence about doing myself.

Thanks in advance.

Last edited by C_Red90; 07-05-2014 at 09:11 PM.
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Old 07-06-2014
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Love a good mystery, but sorry it is your vehicle.
My semantics are:
Cranking engine means starter is turning the motor.
No fire, or no start means engine is not starting up while cranking

OK, first up, the TPS(throttle position sensor) runs on 5volts supplied by a power supply in the computer(PCM).
3 wires are:
5vDC(from PCM)
Return(to PCM)
Ground(from PCM)
When throttle plate is closed you should have .6v to .9volts on the return wire(center wire), above 4.5volts = Full Throttle.

So not sure how you measured 12volts???
I would retest, are you sure it was the TPS and not the IAC valve(IAC does have 12v)

The TPS is a "learned" sensor, when you turn on the key the PCM looks at the TPS voltage, it assumes foot is off the gas pedal so the Return voltage, 0.9v , is assumed to be closed throttle, if it was 0.7v it would assume that is closed throttle, it doesn't care really.
Parameter is 0.6 to 0.9v if it is outside of that PCM will turn on CEL(check engine light).
Same with Wide Open Throttle(WOT), there is not set voltage, anything above 4.5v is WOT.

There is a sub-routine in all computer controlled engines.
The "Clear Flooded Engine" routine.
This routine allows the driver to crank the engine with the fuel injectors OFF, to.........clear a flooded engine, lol.

To activate this routine you turn the key to ON(run)
Press gas pedal to the floor and hold it there
Crank engine(key turned to START)

The engine should just crank now, no fire because there is no fuel, injectors are off.
If you let off the gas pedal at anytime the injectors should start and engine should fire up.

I bring this up because IF your TPS circuit is fouled up, PCM may be seeing above 4.5volts with your foot off the gas pedal so is turning OFF the fuel injectors and you get a no start.

Resetting PCM(pulling fuses) resets the TPS circuit.

As for the fuel pump, possible but a long shot, easy test is to listen for the fuel pump each time you try to start the engine.
Turn key ON, listen, fuel pump will run for 2 seconds, then shut off, if you are not sure turn key off and then ON again and listen.

There are 2 relays that can cause a no start, but engine will still crank.
PCM(EEC) relay, when key is turned on this relay closes and sends power to, fuel injectors, MAF sensor, IAC Valve and Fuel Pump Relay(not fuel pump).

The Fuel pump relay is sent power(12v) when PCM relay closes but has no Ground so doesn't close, the PCM controls the ground.
The PCM grounds the fuel pump relay for 2 seconds when key is first turned on, and then grounds it again only when RPMs are above 500, engine is started.
This is a safety feature in case the engine stalls from a broken fuel line, i.e. fuel pump will not keep pumping fuel out and possibly starting a fire.

Both relays are in the power box under the hood

Intermittent relays are not unheard of.

Fuel filter wouldn't cause your issue, but good to change it.

Does engine fail to start only when warmed up, or will it not start when cold?

Have you ever gotten any trouble codes?
If so what were the exact numbers?

Last edited by RonD; 07-06-2014 at 12:30 AM.
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Old 07-06-2014
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RonD: Thank you for getting back to me so soon, and at such a late hour.

I have tried to pay attention to whether the problem occurred more often when the engine is warm or cold, but it does not seem to matter. Some days it will simply not start even if it has been sitting all night. Some days I will wake up, drive it 2-3 times and think it will be fine after sitting for 5-6 hours but then it will not start and I'll have to come back the next day and try it again. Sometimes I drive it 3-4 times in a row for errands or what not, and it'll just not start when I'm ready to head home. Lately I've been keeping two sets of keys with me so I can leave it idling in the parking lot when I need to get things done.

I have never had any CEL codes other than one time where I accidentally started the engine with 5 cylinders, which happened a couple weeks ago so that didn't contribute to the issue. When I went to the shop that charged me $150 for diagnostics (ridiculous) they said they found a code for IAC and so I replaced that. I have driven the truck for 10+ miles at a time a couple times just to see if the PCM will come up with anything but it does not.

Regarding the 12v reading, I believe I misread some instructions I had found previously about testing TPS connectors and had put the negative prong in the TPS ground and the positive prong on the battery positive terminal. I will have to get back in there and test them the correct way. However both of the TPS modules I posses right now trigger the same readings from the diag tool.

Regarding the two relays you mentioned, I've located them in the wiring diagram (Haynes) and am fairly certain I can find and test them in the vehicle. However with the fuel pump relay, how does the PCM ground it? I don't see any connections to a ground near it in the circuitry. Does it borrow the PCM power relay's ground?

Finally, regarding installation of the TPS...I've noticed that if the throttle position were to properly read 0-1% at idle/off, it would require the TPS to fit square onto the throttle body. However, to install the part it requires a roughly 1/8th turn of the '****' that it fits on to. Is this standard for all of these engines? It seems like it is designed this way. If that is the case, the idling throttle for all 4.0sohc engines is 17-19%? If it helps any, I've noticed that lately the engine has been starting up at a higher rpm, although that may be because I've switched to a higher octane of fuel and a fuel treatment for the moment.

Thanks in advance.
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Old 07-06-2014
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One of the fuel pump relay's "coil" wires(85/86) runs to the PCM, the PCM has several Grounds it uses, it Grounds each Coil to charge it for firing, it Ground each fuel injector to open it, these all use separate ground connections near the PCM.

The PCM close a circuit inside itself, connecting fuel pump relay's coil to a PCM Ground wire, this completes the coil circuit and relay closes.
The power going to the fuel pump also goes to the PCM, it monitors if Fuel pump relay closes, so should through a code if it doesn't.


Yes, there is a popular test for the ground wire on the TPS, if you connect red probe to battery +, then connect black probe to TPS ground you will see 12v..........from the battery, you are just check the ground wire on the TPS harness.

I am not sure what the % are on the TPS, I use sewing needles and pierce the 3 wires, then turn on key and test voltages.
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Old 07-07-2014
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I will get the voltages on the TPS next chance I get, and I just ordered a vacuum gauge to test the vacuum in the engine. While driving today the engine started to stall at an intersection (it was dropping down to 400-500rpm randomly) and I had to gas it in neutral to keep it from stalling completely. I'm pretty sure this is a vacuum issue, so I will use the website you gave me to make a diagnosis and repair accordingly.

Since the carb cleaner method didn't reveal anything wrong with the hoses, I'm assuming the leaking is occurring somewhere in/out of the engine itself through a gasket?

I'll report back with my results when I can get a chance to complete all these tasks.

Thank you
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Old 07-09-2014
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I acquired the readings, they all seem normal. 5v from the PCM, a range of .97v - 4.67v returned to the PCM, and the ground measures 22mv (not sure if that reading matters).

Should I try adjusting the throttle stop so that the range ends exactly at 4.5v?

I'm starting to think that the "absolute throttle position" does not factor into what the PCM sets as the zero-throttle point, however without input from anyone else I have no idea.

Also, for reference when I get the vacuum gauge, where is the best place to test the vacuum with this engine? The gauge I ordered comes with a few different attachments, including a t-shaped one.

Thanks
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Old 07-18-2014
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Ok so before performing the vacuum test today I decided to clean the throttle body entirely (removing it from the engine) instead of just cleaning what I could reach.

There was a little bit of gunk I had not cleaned, plus I made a (seemingly) startling discovery: there was a significant gap between the throttle plate and upper and driver's side portion of the throttle body. I have attached a picture. Keep in mind I tried to tighten the throttle screw to eliminate some of the gap, but it had almost no effect and so I returned it to its original position.

I know these throttle bodies are prone to warping, is this what has happened and is this what is causing the vacuum-leak like symptoms I have been experiencing?

I have decided to hold off on the vacuum test because all of the vacuum ports are a pain in the *** to reach, and I feel like this is contributing to the issue.

If anyone has any tips on testing the vacuum in this engine, it would be greatly appreciated. I have read I need to plug the evap canister, which is above the fuel tank in the '02 model, but can't I just unplug the ignition coil after I have relieved the fuel pressure (for the cranking vacuum test)?
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02 sohc with random no-run condition-img_20140718_175847_567.jpg  
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Old 07-18-2014
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The TPS voltage range is a "Learned range", it isn't exact, PCM learns it during driving.
.97v - 4.67v is fine

You shouldn't have to plug the EVAP system when engine is running as long as gas cap is on.

Follow the power brake booster vacuum hose, it should go to the vacuum manifold, several smaller hoses will connect there.
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Old 07-21-2014
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Thank you RonD for the advice. The brake booster port in the manifold is an obnoxiously simple way to plug the gauge in.

After completing the running engine test, I made a couple discoveries. First off, the "shuddering" that I was experiencing happens about every five seconds during idle and results in a drop of vacuum of about 1/10-3/10 in. Hg. The shudder does not exist at 2000 or 3000 rpm, at these rpm's the vacuum increases slightly and levels out. Likely just a simple vacuum line leak I will have to find, correct?

I also noticed that a test for piston rings said to open throttle and that the vacuum should drop to 5 in. Hg. However with my engine the vacuum dropped to anywhere between 0 and 2. Would this indicate worn piston rings?

Considering my issue, I will probably find a day I have a little more time and try the cranking vacuum test, since this is where my trouble has been coming from. However lately I have not had the no-start issue, but until I find a concrete reason for why this could have been happening and share it with the forums, I'm not going to accept simple "band-aid" results that might leave me stranded out in the middle of nowhere :)
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Old 07-22-2014
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I would unplug the IAC valve on the next vacuum test, see if the shudder goes away, could just be the computer making adjustments.

You need to have a starting point when interpreting numbers for vacuum.
If your warmed up idle shows 20 and you open and close throttle quickly the vacuum should drop to about 3-5, BUT if you open throttle too much it can drop to 0.
The pistons pumping air out of the intake create the vacuum because they are trying to pump out more air than is available with throttle closed.
When throttle is opened all the way(WOT) you will have a second or two when the air the pistons need at that low rpm is available so 0 vacuum, so you don't want to open throttle full, just open it, say half way, and close it quickly.
If rings are worn then the pistons would not be pumping as much air and your vacuum at idle would be lower to start with and so 0 vacuum would show up even at half throttle.

When throttle is then closed the vacuum should go above 20(starting vacuum) for a second as the RPMs are still higher than idle so pumping more air, then vacuum would settle back to "normal", 20.

With vacuum gauge hooked up and showing a steady reading remove 1 vacuum hose at a time from the intake and plug the intake hole with your finger, if vacuum increases you have a leak on that hoses system.
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Old 07-27-2014
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Ok - so probably not the rings.

After weeks of no-hassle start-ups the truck wouldn't start for me again the other day, and after a 20 minute battery disconnect it started right up.

I'm almost fairly certain this is an electrical/computer problem ... Could possibly be crank or cam sensor. While the vacuum leak seems apparent, the vacuum hoses are really a pain to move around ... the ones by the intake are flush against coolant lines. And I don't think a vacuum issue is causing the truck not t start.

Would the warped throttle body be causing a problem?

When it wouldn't start the other day, this time it would reach 500-1000 rpm, but couldn't maintain it, even after repeatedly pumping the gas and doing the injector shut-off procedure. Usually when it starts up it gets to about 2000 rpms, moves down to roughly 1200 till warmed, then down to 600-800 for idle.

Also occasionally the truck will idle at 1000. This gets rid of the shudder but it's bad for MPGs.

Also, still no signs the PCM has a problem. I am considering having the dealership update the PCM and inform me of any critical TSBs.
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Old 07-30-2014
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I have a 2000 4.0, and it was doing the same thing. It looks like it is the fuel pump in my case. (when not working: no fuel pressure; tested the relays, tested the volts at the inertia switch, Tried giving the the fuel pump 12v from the connector of the inertia switch. Everything worked other than the pump.)
My trouble is no longer intermittent, so I am planning to try to pull the tank this weekend.
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Old 08-27-2014
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Sorry for not replying for a while, I was out of the country. A day before I left, I fixed the issue I was having with the Ranger.

After taking a closer look with a stethescope I discovered that when I pinched the PCV elbow directly connected to the intake manifold it created a large air leak. I took the truck up to autozone and ripped the elbow out.

The elbow sits directly on the fuel rail, on a somewhat smooth edge. However this edge ended up creating a hole in the elbow through a combination of friction over time and natural degradation of the hose. The hole was rather large, and after ripping the elbow open the entire inside was cracked and falling apart.

I bought a slightly longer elbow with the same diameter (the only one I could find to fit at AutoZone, it was originally designed for Ford turbodiesels) and cut it down to the 3"x2" specification and the truck runs perfectly.

Thanks for all the help though, at least my truck runs nice and clean now!
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Old 08-28-2014
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When that elbow went bad on mine, I found a silicone elbow on line that worked great. I didn't even have to use hose lamps on it. Recently, I checked it out and the fit has relaxed a little so I will be installing hose clamps soon.
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