94 K1500 starting issue

[Tyke]

I am new to this site but familiar with board etiquette (i hope)

I have a 94 K1500 with around 220k miles.

Truck is in good working order and mostly original. main mod being a flo master muffler.

The truck has always started first time, and run well with slight hesitation from idle and occasional hunting at a stop light.

Last few days the truck started stalling occasionally at full stop. This progressed to occasional stalling on overrun.

Today the truck stalled at a traffic light and would not restart again. The engine turns over but makes no attempt to start.

checked obvious fuses etc. Checked TBI for fuel and it appeared very dry. After around 30 mins and with a friend turning the key, I watched down the throttle body as the engine turned over. Nothing, no sign of fuel. Then suddenly fuel and it started.

Replaced the air filter and headed for home. 5 miles or so later engine stalls on overrun.

Same process followed again, air filter off check for fuel, nothing.

Off to Nappa to get a fuel filter. On return, try starting, fires straight up.

I now have the truck at home and must start the process of locating the fault.

Have searched for threads with similar issues and there seem to be many but non describe the full symptoms I have.

The oil level is good, not sure about the sensor.

Fuel filter although old was also clear and not apparently the issue.

I dont have a pressure guage, but fuel is present at the filter.

The problem is hard to duplicate as the engine runs fine until it doesnt.


Any ideas of what to eliminate first or maybe the best way to diagnose?


Thanks in advance

 

[Erik the Awful]

I would suspect the fuel pump relay, the fuel pump, or a bad wiring connection. A fuel pressure gauge on the backside of the TBI would help. $45 from RVMorse. http://rvmorsemachine.com/product.htm

 

[thinger2]

Most parts stores have a fuel pressure guage in the "loaner tool" program.
Your truck doesnt have a schrader valve so you have to put the gauge inline At the fuel filter
Did you check it for codes?
Not getting fuel at the injectors is not necessarily a pump or relay problem but it could be
A intermittant or heat failing ICM will keep it from pulsing the injectors But will start working again when it cools down.
And ICMs fail on these a lot....
Get an AC Delco spare and some heat paste and a 1/4 drive nut driver with a couple of different size extensions and a 5.5 mm or 7/62 socket and never leave home without them.
thats all you need to take the distributor cap off of a 94 and replace the ICM on the side of the road

 

[Tyke]

Update.

Changed Fuel Relay : Test drive and the fault is still there.

it seems that the loss of fuel pressure occurs when the engine is hot. And then the truck will not start untill the engine cools down.

Today a new symptom has appeared, the check guages light is on and the needles of the oil pressure, water temp and fuel no longer zero when the ignition is off.

Where are the grounds associated with the fuel pump? and can they be accessed without removing the tank?

The fuel pump is not noisy and does prime when the ignition is turned.

 

[Tyke]

Most parts stores have a fuel pressure guage in the "loaner tool" program.
Your truck doesnt have a schrader valve so you have to put the gauge inline At the fuel filter
Did you check it for codes?
Not getting fuel at the injectors is not necessarily a pump or relay problem but it could be
A intermittant or heat failing ICM will keep it from pulsing the injectors But will start working again when it cools down.
And ICMs fail on these a lot....
Get an AC Delco spare and some heat paste and a 1/4 drive nut driver with a couple of different size extensions and a 5.5 mm or 7/62 socket and never leave home without them.
thats all you need to take the distributor cap off of a 94 and replace the ICM on the side of the road

The ICM symptoms you mention sound similar to what I have. I think that is next before looking at the fuel pump ...

 

[thinger2]

T

Update.

Changed Fuel Relay : Test drive and the fault is still there.

it seems that the loss of fuel pressure occurs when the engine is hot. And then the truck will not start untill the engine cools down.

Today a new symptom has appeared, the check guages light is on and the needles of the oil pressure, water temp and fuel no longer zero when the ignition is off.

Where are the grounds associated with the fuel pump? and can they be accessed without removing the tank?

The fuel pump is not noisy and does prime when the ignition is turned.

That sounds more like a basic grounding problem. Check all of the "big three grounds" but start with your battery cables. The factory cables can look great from the outside but will be rotted as hell under the plastic ends.
You can do a real half assed test by getting a bolt that is the right thread for the sidepost. Pull the neg cable, run the bolt in and attach a good jumper cable from the battery bolt to a good clean grounding spot on the engine
See if anything changes.
You can do the same thing to the positive.
Take a real close look at your battery cable ends. You can pry the plastic open a little bit and look inside for corrosion

 

[Tyke]

T

That sounds more like a basic grounding problem. Check all of the "big three grounds" but start with your battery cables. The factory cables can look great from the outside but will be rotted as hell under the plastic ends.
You can do a real half assed test by getting a bolt that is the right thread for the sidepost. Pull the neg cable, run the bolt in and attach a good jumper cable from the battery bolt to a good clean grounding spot on the engine
See if anything changes.
You can do the same thing to the positive.
Take a real close look at your battery cable ends. You can pry the plastic open a little bit and look inside for corrosion

Where exactly are the Big Three Grounds?

On inspection of the battery terminals (which seem to be a good shape) I noticed that the only cable from the negative terminal goes to the engine block, there is no direct ground to the body/chassis. Does that sound correct?

I did the test with a jumper cable from battery to chassis and it did not change anything.

 

[Eveready]

The fact that heat affects it makes me think it is a wire that is just about gone internally and the heat changes things just enough for it to become intermittent. Welcome to the world of chasing the elusive electrical fault. A battery test light is your friend as is a good VOM (Volt Ohm Meter) that can test continuity.

I would look at wires that cross areas of high heat, like near the starter and / or the exhaust pipe or manifolds. The jumper trick will work without taking any existing wire loose. If you bridge an intermittent wire with a good wire (making sure the connections at each end are good) the problem will simply go away. Good luck and let us know what you find.

 

[Schurkey]

You have proven that there's no fuel spray when the truck won't run. You have NOT proven that there's no fuel pressure.

You haven't said anything about checking for spark. Time to do that. You need to find out if there's spark.

No fuel...no spark...common symptom of a failed pickup coil or ignition module. If the ECM doesn't get an RPM signal from the distributor via the ignition module, it doesn't fire the injectors, and you have...no spark, no fuel.

 

[Tyke]

Update #2

First thanks for all the pointers.

Because this is an intermittent fault it is hard to tell when testing what is good and what is faulty.

I checked for spark and that appears OK. Distributor cap and rotor in good shape. I replaced all the plugs with NGK.

While the cap was off, I have changed the ICM unit in the distributor, and while I was in that area I also changed the Oil Pressure sensor.

Long test drive. No stalling ... Result !

I noticed that the engine seems to run much cleaner off idle and no longer surges.

With the new oil sensor the readings on the dash appear different. When cold the pressure shows much higher, when hot on idle the pressure shows lower (but still OK) is that usual?

The dash gauges issue is still there so I now need to find the bad earth.

On another note, while checking wiring under the dash I came across a large after market wiring loom with 4 relays, all wired into the stock loom at various points but activating nothing !

Maybe an old loom for additional lights that have since been removed. The interesting thing was the way they had been spliced into the main loom, the loom wires had been stripped and the new wires simply wrapped around the exposed wires, then electrical tape to cover and a small zip tie to hold in place !

This was clearly done by Pratt and Whitney (when Whitney was on vacation).

If I figure out how to post images I will include some of the loom.