Won’t restart

[Bluetieguy53]

I’ve started working on my 1991 C1500 305 TBI. I’ve done the basic start up repairs after it sat for 15 years. Tune up, oil change, vacuum hoses. My problem is when I first start the truck after it sits for a while it starts great. However after I drive it and park it won’t start unless I disconnect both injector leads. After I get it started it runs until it burns the fuel out in the engine. It will of course shut off. After it shuts off, I attaché the driver’s side injector lead up and it starts fine at which time I then attach the passenger side lead and it runs fine. The engine runs great and makes excellent power. It just has that start up issue. Hope some can help me.

 

[Road Trip]

I’ve started working on my 1991 C1500 305 TBI. I’ve done the basic start up repairs after it sat for 15 years. Tune up, oil change, vacuum hoses. My problem is when I first start the truck after it sits for a while it starts great. However after I drive it and park it won’t start unless I disconnect both injector leads. After I get it started it runs until it burns the fuel out in the engine. It will of course shut off. After it shuts off, I attaché the driver’s side injector lead up and it starts fine at which time I then attach the passenger side lead and it runs fine. The engine runs great and makes excellent power. It just has that start up issue. Hope some can help me.

Greetings Bluetieguy53,

If I can restate your symptoms, your TBI engine starts fine cold, but won't start hot. (Unless you first disconnect
the injector leads and get the engine to burn off the fuel already in the intake manifold?) And other than this the
engine runs well?

This sounds like a specific failure footprint that the TBI system suffers when the ECT (Engine Coolant Temperature)
loses it's calibration & misleads your computer into thinking that your engine is cold when it is NOT, leading to
the computer commanding a way too rich Air/Fuel ratio, flooding the engine when attempting a hot start. (!)

By the way, the next time this problem occurs, instead of disconnecting the injectors I would like you to instead
stomp the gas pedal right to the floor while cranking. When the computer sees the wide open throttle during
cranking it will go into the "Clear Flood" mode. (See 1st attached.) The computer will cut way back on the fuel delivery,
and this will allow your engine to start. If flooring the gas pedal during a hot start works, then this symptom would
confirm to me that you are indeed suffering from too much fuel during hot start up.

The Factory Service Manual has a simple diagnosis involving testing the resistance that the ECT is reporting to the
computer. (See 2nd attached.) NOTE: The higher the resistance, the colder the reported temps. The ECT sensor
can & do fail, but it's also possible for an open wire between a good sensor and your computer to also cause the
misreporting of the engine coolant temperature. (!)

****

We had another forum member run into a similar starting issue, and together we worked through the problem until it was
solved. (Truck won't start, need suggestions) Here's a list of replies where we were making troubleshooting progress:

* Reply #20 (Clear Flood mode - explains why truck will start if throttle held wide open)
* Reply #55 (Troubleshooting)
* Reply #57 (Factory Service Manual procedure)
* Reply #64 (FIXED)
* Reply #71 (Resistance Reading Confirmation)

****

Let us know what you discover. If you are experiencing a similar failure with your TBI system, then the above should work for you.

If on the other hand your ECT resistance measurements match the table (cold & hot) -and- the wire between the sensor and your
computer has continuity, then we need to seriously consider getting a scan tool involved that will show you the live data that all the
sensors are reporting to your computer. And if you have any questions on the above, don't hesitate to ask for clarification.

Since your engine is running fine most of the time, you are close to the finish line. Hope this helps.

Happy Hunting --

 

[Bluetieguy53]

Greetings Bluetieguy53,

If I can restate your symptoms, your TBI engine starts fine cold, but won't start hot. (Unless you first disconnect
the injector leads and get the engine to burn off the fuel already in the intake manifold?) And other than this the
engine runs well?

This sounds like a specific failure footprint that the TBI system suffers when the ECT (Engine Coolant Temperature)
loses it's calibration & misleads your computer into thinking that your engine is cold when it is NOT, leading to
the computer commanding a way too rich Air/Fuel ratio, flooding the engine when attempting a hot start. (!)

By the way, the next time this problem occurs, instead of disconnecting the injectors I would like you to instead
stomp the gas pedal right to the floor while cranking. When the computer sees the wide open throttle during
cranking it will go into the "Clear Flood" mode. (See 1st attached.) The computer will cut way back on the fuel delivery,
and this will allow your engine to start. If flooring the gas pedal during a hot start works, then this symptom would
confirm to me that you are indeed suffering from too much fuel during hot start up.

The Factory Service Manual has a simple diagnosis involving testing the resistance that the ECT is reporting to the
computer. (See 2nd attached.) NOTE: The higher the resistance, the colder the reported temps. The ECT sensor
can & do fail, but it's also possible for an open wire between a good sensor and your computer to also cause the
misreporting of the engine coolant temperature. (!)

****

We had another forum member run into a similar starting issue, and together we worked through the problem until it was
solved. (Truck won't start, need suggestions) Here's a list of replies where we were making troubleshooting progress:

* Reply #20 (Clear Flood mode - explains why truck will start if throttle held wide open)
* Reply #55 (Troubleshooting)
* Reply #57 (Factory Service Manual procedure)
* Reply #64 (FIXED)
* Reply #71 (Resistance Reading Confirmation)

****

Let us know what you discover. If you are experiencing a similar failure with your TBI system, then the above should work for you.

If on the other hand your ECT resistance measurements match the table (cold & hot) -and- the wire between the sensor and your
computer has continuity, then we need to seriously consider getting a scan tool that will show you the live data that all the sensors
are reporting to your computer. And if you have any questions on the above, don't hesitate to ask for clarification.

Since your engine is running fine most of the time, you are close to the finish line. Hope this helps.

Happy Hunting --

Thanks a million Road Trip. As soon as I do the things you told me I will give you the results.

 

[phantom 309]

swap out the ect,. if its running really rich it'll eat the cat(s)
you can change without draining coolant if you are quick,. just do it cold,. loosen rad cap first,. i unscrew untill its ready to fall out with one hand,. and stuff the new one in quick with the other hand thats holding the new one,. yrrmv.

 

[Bluetieguy53]

swap out the ect,. if its running really rich it'll eat the cat(s)
you can change without draining coolant if you are quick,. just do it cold,. loosen rad cap first,. i unscrew untill its ready to fall out with one hand,. and stuff the new one in quick with the other hand thats holding the new one,. yrrmv.

Thanks phantom 309. Will buy a new one asap.

 

[Bluetieguy53]

Thanks a million Road Trip. As soon as I do the things you told me I will give you the results.

I was able to apply the clear flood procedure and it will not start. Strong smell of fuel under the hood. Ad soon as I removed the injector leads it started right up.

 

[HotWheelsBurban]

swap out the ect,. if its running really rich it'll eat the cat(s)
you can change without draining coolant if you are quick,. just do it cold,. loosen rad cap first,. i unscrew untill its ready to fall out with one hand,. and stuff the new one in quick with the other hand thats holding the new one,. yrrmv.

Does that trick work on the Vortec 5.7? I have suspicions that I should replace that sensor in both trucks. But since virtually everyone here at the RV park has pets(mostly dogs that they walk frequently) I'm anxious about doing that repair here and losing coolant on the ground (that I'll have to dilute before someone's pooch gets into it).

 

[docstoy]

Trick will work as long as it's is located at intake manifold, if worried about coolant have shop towel handy.

 

[someotherguy]

Does that trick work on the Vortec 5.7? I have suspicions that I should replace that sensor in both trucks. But since virtually everyone here at the RV park has pets(mostly dogs that they walk frequently) I'm anxious about doing that repair here and losing coolant on the ground (that I'll have to dilute before someone's pooch gets into it).

Engine completely cold, loosen the old sensor and have the new one in hand ready to thread in right as you remove the old one. You'll loose minimal coolant and will be easy to wipe up with paper towels. Definitely understand the worry about pets nearby; I did the whole intake gasket job on my '94 C2500LD inside the Florida shop where my cats would hang out. I started the job outside by siphoning the coolant out to a point where it should be lower than the intake, then drove the truck inside to do the repairs. Knowing it would be impossible to not spill at least some coolant when pulling the intake, I was ready with some paper towels. Ended up maybe with a couple drops on the floor, easily handled.

You must be registered for see images attach

Supervisor Shorty watching to be sure I got it right.

You must be registered for see images attach

All that said, I'm in agreement that it does sound like the problem with OP's truck is the coolant temperature sensor - and it can certainly be tested as described, before just swapping it out. However, if it's old and no history of it being replaced? I'd go ahead and swap it out. They go bad gradually and you may not notice it's gone bad until it's REALLY bad, fattening up the fuel mixture so much that it's wasting fuel, diluting your oil, and killing the catalytic conveter.

Richard

 

[Schurkey]

Yes, the sensor COULD be defective. It could also be a failing wire harness--shorted to ground, high resistance in the wires or the connection points.

Test before replacing. Saves on spilled coolant.