jkemp101

I am running out of ideas for what is wrong with my truck. I have a 1992 full size Blazer with a 350. It died while I was driving to work last week. It currently turns over but seldom fires off. Every once in a while it will start and run for 30 seconds. And then it dies. I replaced the intake manifold gasket about a month ago. It ran fine until it died. I replaced the cap, rotor, wires, and plugs when I did the gasket.
Here is what I have done so far. When it does run for 30 seconds it appears the fuel stops before the spark. The injectors have a nice spray, then the spray becomes less pronounced, turns into drips and then stops. I replaced the fuel pump and rebuilt the TBI. I thought I might have gotten some dirt in the lines while I did the intake manifold. As it turns out the TBI is pretty simple inside and it all looked fine. Spark always looks good.

I have dug pretty deep into the wiring. But to simplify things I have jumped the fuel pump by powering it at the inline fuse. The new pump should have plenty of pressure. I have also given each injector 12V and they spray nicely when I have the fuel pump jumped. I have tried cranking it with a multi meter on the ECM supply to the injector. The DC range didn’t really show much activity but that probably makes sense since it is most likely a PWM output. The Hz range on my fluke showed 12 Hz but of course I don’t known if it is a solid ground. I am guessing the ECM is not telling the injectors to open.

Is there an easy way to check if the computer is sending the injectors the right signal? The ECM only reports code 12 so I guess it thinks everything is OK. Could some ignition problem keep the ECM from opening the injectors? As far as I can tell the fuel system will do its job if the ECM tells the injectors to open. I hate replacing parts without diagnosing the real problem.

RonH

You didn't mention that you have checked / changed your fuel filter. ??? Have you put a fuel pressure gauge on it to see if you have the proper pressure?

jkemp101

I replaced the filter on the pump and the filters on the injectors looked fine. I haven't put a pressure gauge on it because it is so difficult to get one on. The fuel pipes come up over the back of the engine and right into the TBI not leaving much room to get a gauge in there. I am assuming I have pressure since the old and new pump do the same exact thing. It should basically be a straight pipe between the pump and the TBI.

chrisk1500

You test fuel PSI by taking the filter off and installing the gauge there (under the truck on the framerail).....

Spec is 9-12 psibut personally, I would not accept anything less than 11 psi.....

RonH

With TBI the fuel pressure is only 9-12psi?

chrisk1500

Yes....

jkemp101

I think it is fixed at least it passed the test drive around the neighborhood. It appears to have been the ignition module. The basic outcome of all the research is you can have fuel pressure and spark but if the ECM does not think there is spark it won't open the injectors. Below are some things I think I figured out for any future readers with similar problems and a 1992 Blazer like vehicle. It might be somewhere else in the forums but here it is for what it is worth.

I spent a lot of time looking at the fuel supply. The fuel pump has three potential power sources: ECM via fuel pump relay, oil pressure switch above 4 psi, and the Hot Fuel Handling Module. My guess on how all this works is the Hot Fuel Handling Module (and maybe the ECM) prime the lines by running the pump from 2-30 seconds. If you move the switch to run and not start this basically takes effect. The pump will stop after that until you move the key to start. The next step is the ECM detects spark via the ignition module and THEN opens the injectors. The ECM probably keeps the relay energized for some time and then de-energizes the relay. The oil pressure switch should now be above 4 psi and supply the power to the pump for normal running conditions. Remember ECM does not detect spark then no fuel will be supplied. A post by gmcowner helped with me figuring some of this out. It is also easy to do a simple test of the fuel pump by jumping the battery positive to the inline fuse located on the firewall. You can also easily test there with a multimeter to see if power is being supplied to the pump. Just look for +5. The inline fuse should have two gray wires going into it.

I figure the ECM did not throw any codes because from the best it could tell the engine was not turning over. So from its perspective everything was fine.

sargediver

I have the same vehicle w/a similar,but slightly different problem. Truck has 280k miles and suddenly has doubled fuel consumption. I'm on the road as we speak. My first thought is clogged injector(s), as primary fuel filter is only a few months old, and fuel pump is only a few years old. Now I'm experiencing a stutter when I step on the gas at low Rpms, but once past that initial start it's fine. The fuel system is a new area to me and I don't want to start replacing unnecessary parts. I'm guessing my first course of action would be to check the fuel pressure and primary filter, then move to the injectors themselves. If it is the injectors(s), can they be bought independently, or is it a manifold? Specialty tools required? I'm not a professional mechanic but have pretty good mechanical knowledge. Any knowledge/advice is appreciated

Teuffelhunden

Sarge check your plugs. Old plugs will cause the problem you are having they foul at idle or low RPM and it takes the additional heat from acceleration to unfoul them. Does your truck start and idle fine? I had the same trouble as JKemp101 I replaced the fuel pump twice the solenoid, ECM, inline fuse come to find out it was my idle air control valve a cylindrical canister on the passenger side of the TB angled toward the front.

Teuffelhunden

Injectors can be purchased individually but unless you take them out and fool with them or run really old fuel or trash thru the system they are pretty hardy and don't fail easily. Start with the plugs. Start your truck and let it idle to temperature then shut it off and remove the plugs. They all should be beige to dark brown in color if they are black and appear wet they are fouling. When replacing don't fall for the "iridium gold E3" stuff your engine doesn't make the kind of compression needed for these hotter plugs. I use simple AC Delco or NGK plugs. People may object but I have built 3 twin turbo LS setups for people now so I believe I know a thing about combustion. Also a can of "Sea Foam" thru the fuel system works wonders I have actually poured it directly into a carb with the engine running to clean the valves it really is amazing stuff but if you do use it check your fuel filter because it will break a lot of junk in your fuel line loose.