'88 Sierra will not start after stalling

[fixitaz]

OK, Sorry for the book but here's my problem:

Big Red: 1988 GMC Sierra, Extended Cab 4x4, 350ci/5.7ltr TBI-model 220, w/Automatic Trans. 140K plus miles.

The truck sits more than it's been driven the last ten years. Before that is was driven out from Mich. to Phx. Driven all over the SW and Mexico. But other than a bad battery, it always started right up, even after sitting for 6 to 8 months.

Recently went out to make a Home Depot run in "Big Red". She fired right up as always. I pulled it from the back yard to the driveway and let it idle like I usually do after having it sit so long. Rinsed it off with a hose and it was running maybe 5- 10 minutes. Just as I was opening the door to turn on the wipers, it flat died. It died quickly like the spark was removed. no sputter, no nothing, it just quit.

So I grabbed my Haynes manual and started to troubleshoot. For ease of reading I will list the order of what I have done. Another thing is all of these parts are original. I have changed things like spark plugs, wires, cap, rotor, fuel filter, etc. at maintenance times, but the fuel system has been untouched. As one should, I tried to determine if it was spark or a fuel problem.

Confirmed the fuel pump fuse is good.

Confirmed the fuel pump relay was operating correctly.

Confirmed the fuel pump was running.

Removed the fuel feed line from the TBI and fired the fuel pump. Looks like a decent flow but at this point I have no way to check pressure. I am told I need an adaptor that goes inplace of the fuel filter.

Checked the fuel filter just to cover the bases, it's clean.

As a test, I pour some gas down the TBI. It fires on this gas until it is gone, then nothing.



This tells me it's a fuel problem and not a spark problem.



I checked for any error codes with my cheapy code reader, Code 12 (the all is well code) is all that it flashes out to me. no other codes.

After looking at wiring diagrams I 'jumped out' the "fuel pump oil pressure switch", (located behind & below the distributor). From reading and surfing forums I determined this will close once oil pressure has been achieved. From what I can tell this bypasses the ignition switch after the engine is running to keep the fuel pump running. It's a safety to kill the engine is oil pressure is lost. Am I correct?

Next was to inspect the TBI. I removed and rebuilt it. I sprayed all the parts down with carb cleaner, and blew out any orifice I could find. I took care to not submurge the injectors but I did spray and blow orifices on them too. They actually 'looked' fairly clean when I removed them. Re-assembled & re-installed but still no change. I did use the old "Fuel Pressure Regulator" spring, as the rebuild kit did not come with one.

I read that the 'Ignition Module' in the distributor sends a signal to the 'ECM' to pulse the injectors and the ignition coil. Now it would fire with gas poured down the TBI, so I didn't think this was the problem. In desperation I changed it out anyway, still no fire.

I ohmed out the injectors. The are at 0.9 ohms, (digital Fluke meter) I believe this is within range. When I checked for voltage at the injectors, my Fluke meter floats between 1.8 to 2.0 VDC. One injector connected and the other connector to the meter when cranking the engine. I am not sure what signal is being sent to the injectors, so I don't know what this reading is telling me.

I talked to a few more people and decided to try a new ECM. Switched the chips from the old ECM to the new, but still no change.

Now I am really running out of ideas. So I buy a new set of injectors and "try one". My thinking being if the new one does work, it will fire and run, but would be very rough. It still won't fire up.

Now I am reaching...

From what I read, the "idle Air Control" just allows air to bypass the throttle plates to control idle speed. But I am not getting any fuel, so it shouldn't matter how much air is being allowed to bypass. Right?



From what I can tell there is something telling the ECM to "not" pulse the injectors, but I have no idea what it is. Or I am way off base in my troubleshooting.



Summation:

I have fuel to the TBI, pressure unkown.

It just cranks over but does not fire.

It will run for as long as the gas holds out when puring some into the TBI.

The 'Fuel Pump Pressure Switch' has been tried, jumped out, disconnected & no jumper, and with it connected. Still won't start. Does this switch make on oil pressure rise??

I even tried disconnecting the injector plugs, removing one injector, replacing the TBI top, then truned it over. I did get gas flowing to where the injector is supposed to be. It did fire with this raw gas dumping in the TBI body.

Does anyone know the voltage signal the injectors are supposed to receive?

0 VDC = close
12VDC = open

Or Visa-versa??

Or other signal??


So now I ask you people for guidance!! What am I missing?? Does anyone have more detailed wirng diagrams or maybe a detailed sequence of operation for the engine starting and running circuits??

Help a brotha out!!

[68haywagon]

Erp...nevermind missed that paragraph...Sorry!

I'll dig my book out tomorrow and look through to see what you could be missing.

to the board too!!

Mike

[Kevin45]

You might be getting gas to the injectors but are you getting a tricklle, stream, or spray. If it is not spraying then the fault is probably in the tank, that being the fuel pump. I would try replacing the filter first and if that does not do it more than likely the pump needs replaced. Common problem for Chebbies. I worked partt time at my nephews garage and he was replacing a pump or two on a weekly basis. A pump can get weak before it goes completely out and not deliver adequate fuel. That is why it will fire by dumping fuel down the throttle body. The way to tell before you pull the tank would be put a fuel pressure gauge on your line and check to make sure it is proper.

Kevin

[ChevyTech]

Originally Posted by fixitaz

After looking at wiring diagrams I 'jumped out' the "fuel pump oil pressure switch", (located behind & below the distributor). From reading and surfing forums I determined this will close once oil pressure has been achieved. From what I can tell this bypasses the ignition switch after the engine is running to keep the fuel pump running. It's a safety to kill the engine is oil pressure is lost. Am I correct?

No
When the engine has oil pressure a redundant circuit supplies power to the fuel pump. This circuit goes through one of the oil pressure sensors. Power is also supplied throught the fuel pump relay.
The truck has a fuel pump relay that supplies power to the fuel pump when:
- The key is turned on engine not cranking – relay applied for 2 to 20 seconds, depending on emissions equipment. Trucks over 8500 GVW and some other vehicles have and electronic Fuel timer module that allows the pump to run up to 20 seconds.
The key must be turned off for a certain amount of time (usually about 30 seconds) before it will prime the system again when turned back on.
- Anytime the ECM receives a distributor reference pulse.

Originally Posted by fixitaz

Does anyone know the voltage signal the injectors are supposed to receive?

With the key on there is power to the injectors. The ECM grounds the other side of the injectors to make them spray fuel. Thing is the ECM pulses the injectors very quickly (many times per second) so a meter is not used to test voltage across them to measure this pulse. There are tools refered to as “Noid” lights which are LED’s for testing to see if the injectors are being triggered.

Originally Posted by fixitaz

Next was to inspect the TBI. I removed and rebuilt it. I sprayed all the parts down with carb cleaner, and blew out any orifice I could find.

If you got carb cleaner down inside the IAC it may be damaged.


Welcome to the board. That was a damn good post. You covered everything quite well.

My first guess would be the fuel pump is bad. The spec is 9-13 PSI and this system needs good pressure to run.

Here is some info I saved from a previous reply I made to a similar post. Hope the links still work.

TBI systems do not have a fuel pressure test port.

The fuel pressure is tested by using fittings to tee in a pressure gauge. I like to tee in the pressure gauge where the fuel filter is located. If the pressure is good, both the pump and regulator are ok. If the pressure is low it could be the pump or the regulator. If the test is done with a method that allows the fuel filter to be in place, a dirty filter can also cause low pressure. The regulator is inside the TBI unit. Stopping the flow in the return line momentarily will test to see if a low pressure problem is caused by the regulator. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure.

The TBI fuel pressure specification is 9 – 13 PSI with the truck running.


Here are some web sites showing fuel pressure test equipment. The first tool gets installed where the fuel filter goes. Most people use the tool in the third site next to the TBI unit.


I you already have a pressure gauge like shown in the second web site below, all you need is the tool shown on the first web site. If not, the third would get the pressure tested for the least money.

http://www.sjdiscounttools.com/ta37650.html

http://www.actron.com/product_detail.php?pid=16174

http://www.actron.com/product_detail.php?pid=16175

http://www.cfm-tech.com/catalog/fuel...er_2940807.htm



The only outher thing I can tink of right now is the computer needs to receive a distributor reference pulse from the distributor module to trigger the injectors.
It is possible for the module to produce spark without the computer getting a reference pulse. A bad distributor module or wiring between the module and the ECM can cause this.

Once again welcome
ChevyTech

[fixitaz]

That's the info I was looking for! Thanks for the explinations. I have the Actron CP7817, (third link) on it's way. Seeing as how I have already rebiult the TBI, including Regulator, I am guessing I will be changin out the fuel pump.

I'll let you know how it comes out.

Thanks again.

[68haywagon]

There's your answer. ChevyTech is a genious very helpful with these trucks!

Mike

[ChevyTech]

Fixitaz – you’re welcome and I will be watching to see what you find. You articulate what you have done and what you understand with great skill.

68haywagon – thanks for the kind words.