My truck ran yesterday, but not today

Hi:
I think I already answered this. Is that correct with the fuel pressure issue?

Yes you did, sorry about the double question. You had told me that either the fuel return line was clogged or the fuel pressure regulator was bad. I used an air hose to blow through the fuel return line and heard a thud come from the fuel tank. I did it a couple of times and heard the noises coming from the tank so I think the line is clear. I also repaced the fuel pressure regulator with a OEM unit. The fuel pressure is now 40psi when I turn the key and the problem still exists. I don't know for sure if the return line is completely unblocked. Is there any way to check? Are there any other causes ?

I just spent about 30 minutes looking through different manuals. One manual says 9-13psi is spec, another one says 35 to 38 is normal. It should drop by 10 psi when started. Is that heppening?

No, the psi stays at 40

I am so sorry about the two different specs I found. Thank goodness for old shop manuals. As far as the drop when started, does it drop at all?

For a split second

I am going to ask another tech for his opinion. I'm not leaving, but adding another person into our conversation. Can you send me the original post you sent with all the symptoms the car has? Even at 40psi, that is above what my shop manual says is okay. Maybe you are right and there is something blocking the return line. Lets see what he comes up with. His user name is Saturntech with some numbers after his name. He is really good with GM vehicles.

Here is my first post

Asked on October 13, 2011 1992 GMC Typhoon Engine Problem with 83000 miles

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GMC Question

Truck Ran Yesterday, But Not Today

My truck ran fine yesterday. It has a fresh tune up, good spark, fuel pressure at 36psi with vacuum hose attatched, and proper timing. I had my truck in storage for 2 years with gas stabilizer and decided to empty the tank and add fresh gas. Now it won't start.

The truck will fire intermittently while trying to start and produces black smoke. The engine is flooding at startup and the spark plugs were a wet black color. I cleaned the spark plugs with acetone and it still runs the same. I noticed that the throttle position sensor was disconnected so I reconnected it and pulled the ecm fuses to clear the code. There was no change. Unfortunately, I cannot get the truck running long enough to set any trouble codes if there is one. Please give me and idea what to check next.

Thanks, I'll get this to him now.

High imakeorphans why was the throttle position sensor unplugged?You said you plugged it back in?Reset the codes?Do you have any codes now?

Also have you checked the coolant temp sensor yet?

Hi Saturntech, I didn't mean to unplug the TPS, I really don't know how it happened. Unfortunately, that wasn't the problem lol. I reset the computer, but now I can't get my truck to run long enough to throw any other codes. I just sent my injectorswhen out to be cleaned, but when I get them back, I will definitely check the coolant temperature sensor.

Do you know how to test the coolant temp sensor with a multimeter?

Thank you, yes I do. I have my chilton manual that describes the test.

Alright just making sure.

The coolant temp sensor has been replaced, along with the MAP sensor. I should have tested them rather then purchase new ones because it costs alot of money. The problem persists and I keep getting a trouble code 33.

So will the engine run and idle smooth?I posted a wire diagram for your map sensor.ALso do you have a scan tool that displays live engine data like the map sensor reading?

The engine starts but it runs terribly, and with alot of smoke before it will stall out. I have a datamaster cable installed in my truck but I do not have the program in my computer.

We really need to see what voltage the computer is reading on the map sensor that would really help find the problem.

Ill get that for ya real soon. By the way, I accidentally flag your last post for review. Sorry, my phone is a little touchy

The code is for map sensor high voltage so the computer thinks the engine is under a greater load then it is thats why the engine is loading up with fuel.Also did you inspect all the wires to the map sensor to make sure the wires arent shorted together?Dont worry about the flag for review they will figure out you hit it by mistake.

I tested the ecm wire going to the map sensor,without the engine running,and got approximately 5 volts. I got .2 volts from the sensor. Strange thing is, I'm getting voltage to the ground!

I placed the red lead of my multimeter on the map sensor ground and the black lead on the outside cover of my alternator and got 2.35 volts. The wires to the map s. look intact but perhaps voltage is bleeding somewhere. Should I just run a new seperate ground to my sensor or try to locate the short?

So your getting voltage to the wire that goes back to the ecm that shows it as a ground inside the ecm?If thats the case then there is your problem if you run a new wire it has to go from the ecm ground like shown in the diagram and then back to the sensor.Not just to chassis ground or you can just try to see where there is a partial connection between the 5 volt refrence wire and the ground coming from the ecm.So iam correct on what is going on with which wire you found with voltage?

You were right. I found 4.97 volts coming from the ecm according to the diagram. My Chilton manual says that a low .2 volts from the map signal wire is normal.

I'm worried about this ground wire because according to the diagram, that is a COMMON ground. What else, besides my map sensor is recieving voltage through this ground wire

Alright the grey 5 volt refrence to the map splits off and also feeds the tps and the linear egr valve 5 volt refrences.The ground to the map sensor only splits off to the coolant temp sensor the purple wire to it.See if the purple wire to the coolant temp sensor also has battery voltage to it.I also posted a couple of wire diagrams so you can see how the wires split off and where they go.

The coolant temperature sensor also has voltage on the ground. This voltage is how the computer determines the temperature I believe. Am I correct when I say that, according to the diagram, the MAP sensor is supposed to have voltage on the ground?

Why would you have power to the ground side of the map sensor are you checking these wires with the sensors unplugged i hope?The purple wire comes straight from the ecm it shows that wire is grounded inside the ecm.Then it splits off to the purple wire to the coolant temp sensor.The grey wire to the map sensor shows it as a 5 volt refrence so that should have power then it splits off to the tps and egr.The power to the wires that are suppose to be ground would explain the problem you had with the map sensor and the coolant temp sensor which you replaced.

I guess I'm a little confused about how the computer can read the coolant sensor if it is only sending a reference voltage out and not receiving a signal back from the sensor in the form of voltage.

The coolant temp sensor has a ground feed thru the purple wire and the yellow wire is the sensor signal to the ecm.Based on temperture the coolant temp sensor changes resistance so that changes the ground value from the purple ground wire so that way the ecm knows what temp the coolant is.I hope that makes it clear for you how it works.

That explains it. I didn't unplug the coolant sensor when I tested the MAP sensor ground. The voltage I got at the MAP ground was actually the coolant sensor's ground feed because their grounds are connected. I guess I will move on and test the Throttle position sensor.

Were getting a code for high map sensor voltage i would start there with that code.

I decided to attach a vacuum hose to my FPR and reduced the fuel pressure to 30 psi. The truck started right up and idled very smoothly. I reset the trouble codes to see if code 33 remained and it didn't. I got a new code 45 for a rich condition but that is redundant. I will test the vacuum supplied to the FPR to make sure there are no restrictions and then I will run datamaster to see what the computer is reading. I have not learned how to read a datamaster readout yet, could you please help me to decipher? I think the computer is holding the injectors open too long and datamaster may tell me why.

Alright a couple of things here lets get back to your map sensor what ever happended to that?Also with all the sensors plugged in but the map sensor do you have the 5 volt refrence and the ground wire being ground like you should?If not we need to start there.As far as your fuel pressure regulator what is the vacuum reading to it at idle?

The map sensor has been replaced. When the MAP sensor is the only sensor unplugged, there is a 4.99v reference to the MAP sensor, a reading of .02v from MAP signal, and approx. 2-3v on the ground. the 2-3 volts is coming from the coolant temperature sensor because it is connected to the MAP ground.

The fuel mpressure regulator reads 40psi at idle with the vacuum hose attatched. I set the FPR to 30 psi to get the engine running.

The ground wire to the map sensor shouldnt have voltage to it period sounds like the power is shorted into the coolant temp sensor signal wire and is comint thru the coolant temp sensor.In the wire diagram it doesnt show a 5 volt refrence coming from the ecm to the coolant temp sensor.It shows a refrence wire the way the coolant temp sensor works is one side of the sensor has ground from the ecm.That goes thru the coolant temp sensor and as the sensor changes temperture from coolant it changes resistances value.So the resistance change is inputed in thru the other wire to the coolant temp sensor the signal wire.That goes back to the ecm so it can use that change to tell what temp the coolant is.So there shouldnt be voltage there.Because that would throw off the coolant temp sensor reading and the map readings.Like i said before that would explain the map and coolant temp sensor issues you had with it.

Alright i must have been half asleep this morning when i said there isnt voltage to the coolant temp sensor the one wire is a shared ground and the other is a 5 volt refrence from the ecm.Thats what i get for just waking up and answering questiins while iam in bed.Do you you have a scan tool that can read live engine data on that car?

Don't worry about it.
I have datamaster and will be using it to get a live readout of what the ecm is doing. Hopefully I can pinpoint the signal that is throwing the computer off and causing it to flood the engine. I will send the printout when I get it.

Alright get me one of it idling and with the key in the run position and engine off.

Hi, I just wanted to give you a quick update. Upon further inspection, I found out that the intake tube that goes from my air filter to my turbocharger was cracked all the way around. I replaced it with a cold air intake system and started the truck.

The truck runs perfectly!! No need to reduce the fuel pressure. I am not sure if the intake could not have caused the problem I was having, but my truck has been running fine for the last 2 days so I will won't worry about the problem until it comes back. Thank you for all your help, you have kept my sane through all of this!