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91 TBI 350 Shuts down when hot but will start after cooling for a few minutes

JPOutfitters said:
and the oil pressure switch is only for cutting out the fuel pump at a loss of oil pressure. After replacing the oil pressure switch I've noticed a more constant reading at the gauge now!:rolleyes:
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Just an FYI, the oil
pressure switch only bypasses the fuel pump relay to power the fuel pump if the relay fails. It does not cut power to the pump if the oil pressure gets too low. It doesn't matter in this situation though.




-Brian
 
bp71k5 said:
The oil pressure switch wasn't the problem. It doesn't ever shut the engine off, it only keeps it running if the fuel pump relay dies.



-Brian
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I'll see if I can find where I read that the oil pressure switch is designed to cut off the fuel pump at the loss of oil pressure as in when you shut down your engine so that if you get into an accident and the engine stops it will shut the fuel pump down instead of continuing to pump fuel......the fuel pump relay allows the pump to run for a small amount of time upon initial startup and engine cranking to allow for the oil pressure to come up at which point the oil pressure switch will then keep the pump running!

It is my understanding that if you disconnect the fuel pump relay while the engine is running it will continue to run, but if you disconnect the oil pressure switch it will shut down the fuel pump! Which is why those individuals that are having issues with the vehicle starting up and running for 20-30 seconds and then dieing is because of a faulty oil pressure switch!

:dunno::dunno::dunno::dunno::dunno::dunno:
 
JPOutfitters said:
It is my understanding that if you disconnect the fuel pump relay while the engine is running it will continue to run, but if you disconnect the oil pressure switch it will shut down the fuel pump! Which is why those individuals that are having issues with the vehicle starting up and running for 20-30 seconds and then dieing is because of a faulty oil pressure switch!
Click to expand...

I had the same confusion for a bit until i was schooled on TBI workings.

If you unplug the relay, the motor will only start after it builds up
3-5 psi of oil pressure. It will also keep running without the relay.

If you unplug both the relay and the switch then it'll die and not start anymore because at least one of those items is required to be working for the fuel pump to get power.

If you can get a wiring diagram, looking at the way they are interconnected helped me understand it better.

-Brian
 
bp71k5 said:
I had the same confusion for a bit until i was schooled on TBI workings.

If you unplug the relay, the motor will only start after it builds up
3-5 psi of oil pressure. It will also keep running without the relay.

If you unplug both the relay and the switch then it'll die and not start anymore because at least one of those items is required to be working for the fuel pump to get power.

If you can get a wiring diagram, looking at the way they are interconnected helped me understand it better.

-Brian
Click to expand...

I agree with what you have said, but if you read the link I posted about what the purpose of the fuel pump relay is it describes the fuel pump relay as cutting out once the engine is running and only energizing again when you shut the ignition down......thus allowing the oil pressure switch to maintain power to the fuel pump as long as there is oil pressure. It does say that the oil pressure switch is a "back up to the fuel pump relay" but I don't understand what the purpose of the oil pressure switch would have then, and why I have also read that it's a safety switch to shut off the fuel pump when the oil pressure is gone.

I'm not trying to step on your toes, I'm just basing this off of what I'm reading......and if the TBI system is functioning properly then the fuel pump relay is only supplying power to the fuel pump on initial startup and at shutdown.
 
I would like to see how GM describes how the system functions properly and what the purpose of the fuel pump relay and the oil pressure switch is.

Is there a GM manual that has this listed?
 
I've had several of the ignition control modules go bad without ever setting a code. Just saying.
Look at codes as, symptoms, not necessarily the exact problem.

With GM TBI small block, one should always carry a spare ignition module, in the glove box, and a screw driver to install it with.
 
JPOutfitters said:
I would like to see how GM describes how the system functions properly and what the purpose of the fuel pump relay and the oil pressure switch is.

Is there a GM manual that has this listed?
Click to expand...

Both the chiltons and Haynes have a diagram showing how it works. I'm
not disputing the article since it doesn't describe what type of engine it's referring to, only making sure you knew that in a TBI engine, the switch doesn't cut off power when the oil pressure drops.


-Brian
 
I just ordered PDF Ebooks of the Rochester TBI system shop manuals....both the electronic shop manual and the repair manual for $15.98

I'll see what they say and review them upon download........I just want to make sure I have all the facts straight so that I'm not passing on information that isn't correct.

Thanks for all of the input so far!:pimp:
 
Ok, well you said pump was newer, so that helps.
Now you mention it I remember the hassle you went through with the pump
 
JPOutfitters said:
When this first starting happening I couldn't hear the fuel pump cycle when I first turned the key back on to restart it until I let it sit for 5 minutes or more.....this prompted me to change the fuel pump relay. When it started doing it after the fuel pump relay change I noticed that I could hear the fuel pump cycle immediately after it died if I cycled the key on after 10 seconds. Which prompted me to pay attention to the oil pressure when it died. I noticed that the oil pressure was jumping from 30-60psi but would die when registering both 30 AND 60 psi inconsistently but only when it was hot. Since changing the oil pressure switch it hasn't died and I'm getting a constant 60 psi while the engine is running.
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based on this and the fact that it hasn't died on me yet I think I might have had a combination of a faulty oil pressure switch AND fuel pump relay along with a clogged fuel filter as when I changed it out I had sludge draining out of the old one! I'm thinking that the fuel pump relay would energize when it was cool to start the vehicle but the oil pressure switch was working to keep the vehicle running when the fuel pump relay would short out for heat but suddenly would start to fail itself due to the heat from the exhaust and having both the oil pressure switch and the fuel pump relay inoperable it would shut down the fuel pump. Perhaps this issue is why I wasn't getting a code for the fuel pump relay and also why I wasn't hearing the fuel pump cycle/run immediately after it shutting down and my attempt to cycle the ignition switch off and then back on immediately. After replacing the fuel pump relay I could hear the fuel pump cycle/run immediately after shutdown and since the oil pressure switch doesn't cut out power to the fuel pump by itself it then was the fuel filter issue.

Everything happens in 3s right? Fuel Pump Relay, Oil Pressure Switch, and Fuel Filter.........

We'll keep an eye on it, but after running it for an hour straight today I've had no issues of it shutting down!!:whistle:
 
on another note though...........when this was first happening it didn't act like I had to prime the fuel pump by cycling it a couple of times to restart. I simply would let it cool for 5 minutes and it would IMMEDIATELY start back up on the first try and run normal! No hesitation.......

This would happen at a Full tank and at a 1/4 tank of fuel so I'm assuming this wasn't the fuel pump overheating/seizing.

Electrical can do funny things........:popcorn:
 
63chevyll said:
Makes we wonder why the last guy didn't change filter when he did the pump
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he did, and I changed it again since then to make sure he didn't knock dirt into the tank when he did the pump since he was in my hunting camp fixing his work from the first time!!
 
*update*

Okay guys...........weird again! Did it again, this time I drove all over the woods last weekend hunting Elk and had no problems there, during the hunt, and back. My wife took the Burb to work Monday and no problems. Tuesday she goes to pick up the teenage daughter at the Woodland Park Cross Country meet and BAM, dies! Starts right back up and she parks and then it won't start again.......

So I drive up in the next project vehicle, '88 Grand Wagoneer Limited, and I can't hear the fuel pump cycle on at all. So I go ahead and install the NEW Ignition control Module I've been waiting to install since this is the FINAL piece of the electrical side of the fuel system other than the fuel pump itself left to replace........NO START!

Bang on the fuel tank with a hammer 50 times and guess what? Starts right up........:poo:

SO......

I just ordered an Airtex E8094 Inline Fuel Pump that gives 12-17psi at 45-50GPH and I will be wiring it inline just forward of the tank by cutting a section of metal fuel supply line out and putting in a fuel filter between the pump and then replacing the OEM filter that will be further down the line between the pump and the TBI. This pump is rated for TBI systems and provides a little more pressure than the stock in-tank pump that GM put into this system. After further investigation I've also come across some information that GM set the system up originally to have 11-13psi and anytime you do airflow mods like exhaust, TBI spacer, and free flow intake filter the engine runs a little LEAN because of OEM Fuel Pressure!

This pump run inline in the fuel line outside of the tank will in series with the pump in the tank will boost pressure on average to the 14-15psi area which will more than likely give me more power! Investigation from my readings is that minor airflow mods require that pressure range to keep it from running LEAN and should boost some power and correct my problems.

I will be doing this modification tonight as the pump gets here via Carquest at 315pm today! Next month after hunting season is over I will be removing the tank and removing the OEM pump and putting in a solid piece of MARINE fuel line where the OEM pump goes to utilize the original pickup assembly and float. I'm going to wire the Airtex pump to the electrical supply wire for the OEM pump to keep the system working correctly with the Oil Pressure Switch and the Fuel Pump Relay.

Now I just need to verify the correct color wire going into the pump harness at the tank for positive...........:dunno:
 
I think you may have.....Lord, I just got to.............Hit upon the problem........


My local mechanic has cranked many cars brought in on the wrecker by using a short piece of 2X4 and a hammer to make the pump run long enough to get it on the rack.
My Mercedes shop manager even showed me the best spot to hit the tank on my M-Class.

Some of the things you have read, seem a little doubtful, but I don't think the pump will hurt much.
About the worst thing that might happen is it runs rich.

Remember, the fuel pressure at the injectors is set by the regulator, not the pump as long as the pump can supply whatever pressure at the volume used.
Unless you overload the regulator with volume, that pump should not change the pressure at the injectors much.

But, the question is, can it draw gas through a non-functioning pump?

After you get it hooked up and wired in, it would be interesting to crank the truck and disconnect the wire to the original pump to see if the engine can keep running on just the booster pump.

If it can't, you have not solved the problem........
 
MTChevy said:
Airtex is a bad word...seen waaay too many new ones last under 100 miles!
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with a 1 year warranty and the purchase through Carquest, which has the best customer service in the industry, I won't have any issues!:waytogo:

I put 300 miles on it this past weekend with no issues.:D
 
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