I thought I would give this another go. I'm looking for help with my 3.9l dak. So far I have replaced:
spark plugs
plug wires
dist cap
coil
rotor
dist pickup sensor
map sensor
throttle position sensor
coolant temp sensor
computer
o2 sensor
fuel filter
air filter
and the crank sensor is clicking when i run a wrench past it. I was told this means its working.

The problem with the truck is it starts....runs for about a minute....then shuts off. Sometimes it sputters to its death other times it just shuts off. I have had two mechanics look at it since I couldnt figure it out and they are both stumped with this one. They told me to just start replacing stuff and let them know how I made out. They both hooked up a little diagnostic tool and a fuel pressure gauge.

The fuel pressure hovers around +/-30 at idle, and jumps to around 38-39ish with the vacuum line unplugged. It doesnt drop off when the truck dies to indicate a bad fuel pump, clogged lines, etc....The computer diagnostic tool goes all over the board. I havent had one hooked up since the computer swap last week, but......it still does the same thing so I can only assume its still gonna read the same. When I pull the plugs they are all black and sooty and smell of gas. Im figuring that is from when it sputters to its death. I've just been cleaning them and dropping them back in. When it begins to act up, if you push the go pedal down it dies. Sometimes when I let off the go pedal it recovers. Other times it just starts to sputter and backfire.

This thing sets no codes, and the check engine light never comes on except for when I unplug a sensor.

I have described this to the best of my ability, if anyone has any idea what may be wrong with it.......I would really appreciate it if they could clue me in. Until then the dak is making a nice lawn ornament. The only thing that sucks is I need this truck since the garbage is piling up in my shed. I normally use my truck to get my garbage to the dump once or twice a month, but its been dead for two months now.

 

I remember this. Oh boy...
The list of things you have replaced is... well, a lot.
I can think of two items that PCM either recieves input from or controls that, in some way, may have an effect, that you have not listed as replacing:
Idle Air Control motor - the PCM controls this to adjust the idle speed. (Picture) It moves the metal piece in and out to to control air flow. This is mounted on the back of the throttle body. ($55 at NAPA, $45 at Autozone)
Intake Manifold Air Temperature Sensor - This provides the PCM the air temperature as is in the intake manifold (as the name implies). This is mounted (IIRC) on the trucks right side in front of the throttle body (but not mounted on the TB). You can test this (the same test applies to the coolant temperature sensor):
(1) Disconnect wiring harness from connector
(2) Connect an ohmeter to the sensor ends. Resistance should be as shown in the chart (provided).
You have replaced many sensors that would cause the computer itself to cause this problem.
I would just suggest to go and check the integrety of the wiring (ie. make sure it's in good shape, and the resistance from one end to the other is very low)
If you need info as to wiring and all that, just post back, or PM me.
Remember to double check all grounds, and clean/relocate any grounds that even look suspecious.

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Thanks for the relpy. I spent the better part of my last two sundays ripping all the wire looms apart under the hood and checking the wiring with my mulitmeter. I did find a couple bad grounds. The old computer wasnt grounded very good as well as a couple other things. I found a few shady connections under the fuse/relay box under the hood and a wire was broke off down on top of the bell housing. That fixed my fuel guage that wasnt working.

I will go out right now and check these two out.

I also forgot mention that it has a new throttle body gasket and a new intake manifold gasket. I did notice however that the former owner removed the egr solenoid, and the transducer.....would this have any effect on how this runs? I fabbed up a plate that blocks off where it was on the intake manifold. (They had the valve still there but it was stuck open) But it ran fine (acctually better) for over a month with the plate on there before it started doing this. I wouldnt imagine that not having the egr would create a problem like this, but.........I'm not a pro here.

By the way this truck is mpi. If that helps with anything. Also, the last time the guy came with his data aquisition tool, he said that the auto shutdown kept comming on, killing the truck. I replaced the relays and checked all the wiring. Then bought the new computer.

Thanks a bunch for the chart and the input. It is greatly appreciated.

 

The intake air sensor is dead. I hooked up the leads from my meter and it does nothing like its blown wide open. I'm gonna go get one of these today. Thanks for the info.....I will let ya know how I make out. Also any other ideas would be greatly appreciated.

Thanks

 

Well, the shop manual says a leaking EGR valve could be one of the possible causes for an engine that stalls or idles rough. But, since you said it ran fine for over a month after fabbing the plate in there... it would lead me to believe that that is not the problem.
And all the vaccum lines are hooked up correctly, and in good condition?
Is the timing chain/belt routed properly, and not loose?
I would be thinking we are running out of computer-controlled or electronic things to diagnois/replace, and be leaning more twords possibly a mechanical problem.

 

I went this morning and ordered a intake charge temp sensor. 22 bucks and Ill have it tomorrow afternoon. I put the ohm meter on it and it does absolutely nothing. To make sure my meter was working I hooked it up to a speaker that I know is 4 ohms. It read 3.8. So I went back to the sensor and still nothing.

I have gone over the vacuum lines a time or two. I tought that was the problem at first. I had them all apart a while ago and thought I must of messed something up. The diagram under the hood is for a 92 2.5l 4 banger that the truck came with and thats how I hooked them all back up. (whoops!) The previous owner swapped in this 1994 3.9l. So I had to get on here and someone hooked me up with a diagram closer then the one I had. They were about the same tho.

I have thought about pulling the timing cover off and looking in there to see if things line up. When I asked the mechanic if the timing belt/chain jumping was a possibility while he was here he said no...the way his box was acting it was definitly computer related. He showed me how the computer was confused on what to do after a couple seconds. Readings would go haywire as soon as the truck started acting up.

As for me...I am just good at ripping it apart and putting it back together.....trouble shooting isnt my strong side. I just listened to what he said and kept on tinkerin.

 

Yea, these computer controlled systems are pretty good until your having problems, and sometimes they seem impossible to fix.
Good luck, if I can find anything else that might help, I'll post back. If you find the problem, post back.

 

Will do, and thanks again. I really apreciate the help. Lord knows I havent been making much headway on this.

 

Well I got home and installed the air temp sensor today. Still does it. I am at a loss.

 

Right after it dies, can you start it right back up?
You are sure everything is hooked up correctly?
If you are running the A/C, don't at this time.
Check the computer for codes before you unhook it.
Reset the computer (unhook the negative battery connector and leave it unhooked for about 10 minutes)
Than take off the air filter and take a look at the throttle body plates. Give 'er some throttle body cleaner.
Than, reconnect the battery.
Test the PCV valve (picture)

Try using a motor treatment, such as SeaFoam. You want something to clean the valves, and something to clean the injectors.

Questions for you:
1.) How long after you put this engine in the truck did these problems arise? (Mile wise, and time wise)
2.) Between initally putting the engine in, and when it started acting up, what was done to it? (i.e. did you put a new intake on it? exhaust? etc.)
It seems that you are running out of things, sensor or computer wise, that might make this happen.

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