Gmc 7000 update

Zach Scherf · Jan 15, 2015

Watched the video on reading the codes, can't find where the plug in is located at? Looked all over under the dash and hood and don't see one. The fuel pressure regulator appears to be fine as there is zero fuel in the vacuum line. Going to unplug the o2 sensor and try running it to diagnose if that sensor is bad

Zach Scherf · Jan 15, 2015

Also mechanic who had it a few days ago changed the points coil and condenser and he believed the distributor to be in good condition. Think I addressed everyone's comments, sorry if I missed yours!

DENNIS MIN · Jan 15, 2015

Zach,

Forgive my ignorance, but your truck has points?

I thought GMC had forgone points long ago.

Still interested in your outcome.

D.

Tim GA · Jan 15, 2015

The data plug should be under the dash near the steering column. you need a good scan tool to read the computer data to see what is not normal.
unplugging to o2 sensor will only keep the computer in open loop. For the computer to store a code the sensor must be at the extreme voltage of more than 4.5 volts or lees than 0.5 volts for 2 or 3 seconds. The coolant sensor with ignition key in the run position and not ran for a long time should indicate close to air temp. The map sensor should be close to 1.6 volts with engine running. The map sensor is load sensor for the computer. the map sensor is like a vac gage. you need the data and code to see what is out of range of normal. To check for codes jumper terminals A to B and check the check engine light flash with key on engine off.

INCase · Jan 15, 2015

yeah, no truck (or car) with an o2 sensor would still have points.

what year is the truck?

I presumed since it is a 7000 series it was older (newer ones are 7500's???)

Zach Scherf · Jan 15, 2015

It's a 1989. Forgive my ignorance but what is inside the distributor if they are not called points?

Zach Scherf · Jan 15, 2015

This is the list from the mechanic

INCase · Jan 15, 2015

Never mind I see your post below.

it still would not have points though.

was the GMC an old schwan's truck too? If so weren't they all propane.. would the conversion be wrong??

I've often thought of doing what you're doing but maybe not deliver and probably buy the milk but process raw milk and sell in bottles. We have plenty of agriculture here and just enough yuppies and tourist for customers to make it work i think. ALways liked the way Meyer's dairy in State College, PA did it.

Good luck.

Zach Scherf · Jan 15, 2015

Sorry first time with a picture here it is (I hope) haha

Zach Scherf · Jan 15, 2015

The truck is a former Penske truck and the box is from a wrecked schwans truck. It's a lot of work to do all the delivery. I do all the tractor work and feed the cows. Dad bottles and delivers in the end it will pay off but with no delivery it's kinda hard to make money lol

jimg.allentown · Jan 15, 2015

Inside the distributor, you have a hall effect sensor that takes the place of points. You also have a rotor that distributes the spark to each individual cylinder.

jimg.allentown · Jan 15, 2015

Tim, the O2 sensor voltage should NEVER exceed 1 volt. Normal readings are around .7 to .8 volts on the rich side, and .1 to .3 on the lean side with a reference voltage of .5 volts being the "ideal" target reading.

Sprint 6 · Jan 15, 2015

What year is the truck and what exactly is it doing?

used red MN · Jan 15, 2015

Here is a link to his earlier post that give more detail of the problem and what he has done as well as YT'ers replies.
Earlier post more info

Zach Scherf · Jan 15, 2015

Ok to add to the confusion. There Is NO plug under the dash that you can plug into or use a paper clip to check the codes. Called mechanic and he said that he searched all over and even took the glovebox out and still didn't find it. I unplugged the o2 sensor and tried to start the truck. It didn't start. Plugged the sensor back in, it started so I would venture to say the sensor is good? Not sure if that's exactly good logic but it's what I got. So now the truck is running very bad at idle so we turned up the idle a bit and can get it to run, but very poorly. We assume that perhaps the throttle body/excessive fuel problem has been solved. However, it still runs very poorly. We just tried moving the distributor both ways, and it made a little difference, but still isn't running good. Any ideas now?

Ron-MO · Jan 15, 2015

I am always amazed to see these shops that "parts swap" and bill labor and parts and still no resolve, or reliable diagnosis, but they certainly are not afraid to charge the customer (for nothing). IMHO.

jimg.allentown · Jan 15, 2015

Start with some basics. Check the timing with a timing light as per the instructions on the VECI decal that should be under the hood someplace. If you cannot find the decal, you will need to get the procedure from a manual. Next, I suggest that you check fuel pressure with a gauge. You can get an inexpensive universal gauge from Harbor Freight that will do the job for the little bit that you will need it. If the fuel pressure is too low, it will cause poor running.

Regarding the oxygen sensor: GM O2 sensors are notorious in that era for failing. The reading from that sensor is what puts the engine into closed loop operation. At that point, it operates as a full feedback system. Unplugging the sensor should only cause the system to go into open loop. It should not cause a no-start. Unplugging it does not prove or disprove the sensor. At this point, I would recommend replacing it just due to age and condition. They are not that expensive, and are a key component of the feedback system. Also, O2 sensors need to warm up to operating temperature to begin to function. As I recall from reading your repair invoice, the repair shop listed repairing the wire to the O2 sensor. That tells me that it ran before with the sensor unplugged.

Also, have you neglected some REALLY basic stuff, like how are the spark plugs? If the repair shop replaced them, did they put in ACs? Or Champions? I have seen SEVERAL cases where Champion plugs simply would not work right. They never work well in Ford or GM products, but are fine in Chrysler products. How about the plug wires, and firing order? A couple of switched wires can really cause problems.

Keep checking things, and don't dismiss anything until you are SURE that it is OK. Good luck!

Sprint 6 · Jan 15, 2015

Thanks for the link to the original post. First, as others have mentioned, check the ground wires first, there are usually two sets at the front of the intake or thermostat housing. If the two terminals are stacked on the same bolt, put one on another bolt. TBI systems are very sensitive to the grounds. Make sure they are separate, clean, and tight.

Second, check the distributor reluctor magnet. It will be the piece under the rotor that turns with the engine and has eight points on it. The distributor has to be removed and disassembled for a complete inspection. You are looking for any cracks in the magnet that is sandwiched in between the two metal pieces by rivets. Also check for cracks in the piece with the eight points. Any cracks in the metal piece usually make them not start, but they can do all sorts of silly things. Cracks in the magnet lower the output of the pickup coil and can cause the ignition module to not be able to send an accurate signal to the computer. Any cracks require a new distributor shaft or reman distributor.

Third, make sure the exhaust is grounded good. If the exhaust is not well grounded, the oxygen sensor cannot do its job.

Good luck, this is the best I can do. O2 sensor problems can usually be caught with a scan tool. The distributor problem can only be caught by experience or a scope. BTW, those run higher fuel pressure, 32-36 psi on the 366 and 427 only. I have seen some shops try to tell people they have too much fuel pressure because pick ups run 9-13psi. Good luck.

glennster · Jan 15, 2015

zach, the obd 1 plug plug may not look like a standard plug like a car has. on our 92 international roll backs, it is a round port kind of like the vehicle side of a trailer plug. i had to get a special adapter for my snap on modis scanner and the medium heavy duty software to scan the trucks. the plug is located on the left side lower kick panel, kind of where the emergency brake pedal would be.

the tractor vet · Jan 15, 2015

Well just because there is NO FUEL in the vac line does not mean that the reg is good. Back when i bought my first new high tech injected one ton i had a problem with it and like you i took it to people that were suppose to be in the know . Yea wright , the only thing they knew how to do was lighten up my check book and not fix the problem . Mine was on a 460 Ford and like yours ran rough plum ate gas and smoked like a diesel with the wick turned up . Well i figured it was time i LEARN this new high tech junk. So all customer work came to a standstill . I talked to other people and got all kinds of Well it could be this and it could be that and on and on and still no fix .Bought a fuel pressure gauge and hooked it up and OH MY why is the fuel pressure running so high . Oh what fun job it was to remove and once off and a little testing with a vac. pump and gauge told me that this thing was junk . Took the drive to the Ford dealer and bought a new one and everybody there kept telling me that they don't go bad only if they are leaking into the vac. line . Well mine did not leak into the vac. line but it would NOT let the gas flow back to the tank and was DUMPING it all thru the injectors . Installed the new Reg. and The engine ran like a swiss watch. I also said that i was never buying a scan tool also , YEA wright i now have a Snap On Solus . Along with other special items. IF your going to work on this new junk that is suppose to be SO much better then the old ya have to have it .

Duner Wi · Jan 15, 2015

This won't be much help but you are not alone.

http://www.truckforum.org/forums/gmc-truck-forum/34403-dlc-location.html

Frankmn · Jan 15, 2015

I know if there is too much gas in the crank case they will not run good. Have you smelled the oil to check it for fuel?

GregCO · Jan 15, 2015

Something that stuck out on your shop ticket is I did not see where they checked the fuel pressure or fuel return line. Any chance you could get a fuel pressure gauge on that thing? TBI is really touchy on fuel pressures. I would also check the fuel return line and make sure fuel is returning to the tank. If the return line is pinched somewhere that may throw your fuel pressure way off. When the box was put on it the return line may have gotten pinched.

Greg

Bob · Jan 15, 2015

There is NO "vacuum line' for fuel to leak into on a GM TBI system of that era. You, vet, of all people should know that!

Bob · Jan 15, 2015

Jim, a GM TBI of that era would NOT have a "Hall effect sensor".

For gosh sakes, the OP is CLUELESS and looking for help, and then "experts" post something as silly as it having a "Hall effect sensor"???

Ron-MO · Jan 15, 2015

Zach, if this so called mechanic says he changed points, you need to ask for a refund as far as I am concerned, as unless it has been swapped that distributor has no points. Points went away in the mid 70s last I knew.

Owen Aaland · Jan 16, 2015

1989 was the first year for throttle body injection for the 366. Schwans did not have a propane conversion ready to go for the throttle body engines so if you got a used 1989 Schwans truck it would have a 454 in it instead of the 366.

When you go to get engine parts for a 1989 medium GM truck you will find that the 454 is not an option that year. It just goes to show that if you buy three trucks a day all year long you can get things done your way.

jimg.allentown · Jan 16, 2015

Go back and re-read the original post. He stated several rimes that this was NOT an original Schwans truck. The Schwans box was put onto a DIFFERENT TRUCK.

Super-H-Mike · Jan 16, 2015

Here is what I would try next, unplug the injectors and run it only on propane or carb cleaner and see if it clears up any.

jimg.allentown · Jan 16, 2015

Well, excuse me......
Most distributor pickups ARE hall effect - regardless of how the manufacturer assigns proprietary names to them.
You are right about the guy being clueless. And everybody wants to offer a "short" answer that will make the whole problem go away. No diagnosis, no testing, and just throw parts at it.

BTW, what does it matter what type or name of sensor is in the distributor? It probably is NOT the problem to start with, and should require no further concern on his part anyways.

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