TBI troubles....

[mckustomfab]

1991 Chevy R/V 3500 5.7L tbi. I had swapped in a suburban fuel tank and ran it with no problem. Then I went to work for 5 weeks and when I got home it won't start. And the fuel gauge is showing at the 3 o'clock position.

At first I thought the non starting was because it was low on gas so I added 10gal and still nothing. The fuel pump would not run for the 2sec like it should with the key in the run position. So I pulled the pump and started checking with a multimeter. I have 12v at the fuel pump and I bench tested the pump, it is good. I checked the fuel pump ground and it is good too.

I sprayed a little starter fluid and the engine will run until it is all gone. So this has me scratching my head.

To recap, I have power to the pump, the pump is good and spark at the engine but the pump won't pump... Also I tried to pull the error codes but I'm getting no "check engine" light and when I jump the terminals it doesn't flash anything. Not sure where to start, I don't want to throw parts at the problem so some help would be appreciated.

 

[mckustomfab]

So after having a different and less aggravated set of eyes check the fuses for the 3rd time, we found a popped fuse on "ecm 2". Now I have the check engine light back but not showing any codes. Also I have 12v at the injectors, which I didn't have before I changed the fuse.

 

[mckustomfab]

So I wanted to check to make sure I was getting fuel and I am, up to the tb. It seems like the injectors are not firing, but I can't check the voltage while cranking because my helper left. So I will have to do it later, I might also go get a noid light to be double sure.

Any ideas on what could cause the injectors to not fire? I have read a lot of posts and see things from coil to oil pressure sender.

 

[4xcrazy]

I went through this for quite some time, was slow for me because the truck was 2000 miles away.

Many will probably say to have the ignition control module inside the distributor cap checked or replaced, I did this a few times and still had problems. Mine ended up being a bad connection with a connector plug, that plugs in the ignition coil and goes to the distributor. May have possibly been just the coil, it was an aftermarket unit, but both the wire pigtail and coil were replaced.

So far, it made the long journey from north east Indiana to western Washington and is still working fine, other than other little issues now.

 

[mckustomfab]

So would the coil keep the injectors from firing because the truck will run on starter fluid it's just not getting fuel.

 

[4xcrazy]

Somehow or another, this was the case with mine, I'm still not entirely sure what the deal was, I could always get mine to fire with ether, it's just the injectors were not getting a signal to pulse. I'm guessing the coil was supplying the power to the plugs, but not the signal to the dizzy to tell it when to fire the injectors.

 

[mckustomfab]

Update...

So, I bought a noid test light and a new ignition coil. Tested the old coil and showed it wasn't firing so I swapped in the new coil. Then I hooked up the noid test light and checked for signal at the injectors. No signal to the injectors...

So now my question is, is there something that could be causing this? I have read that something in the distributor could cause this (don't remember what it's called). I'm really hoping it's not the ecm, if it were though would it still run on starter fluid?

 

[readymix]

Pick up cool in the dist.
Broken reluctor ring in the dist.
Failed magnet that sits inside the reluctor ring.

You do know that the injectors are fired by switching the ecm lead to ground right? Just want to be sure you are looking for the right signal with your noid light.

 

[mckustomfab]

Pick up cool in the dist.
Broken reluctor ring in the dist.
Failed magnet that sits inside the reluctor ring.

You do know that the injectors are fired by switching the ecm lead to ground right? Just want to be sure you are looking for the right signal with your noid light.

Yeah I have seen where the injectors use the ground from the ecm. The noid light I have just plugs into the injector and should "light" when cranking.

Guess I will get a new pickup coil next to see if that is it. Thanks for the input guys!

 

[mckustomfab]

Installed the new pickup coil and now I have the injectors firing again! But it still won't start.... I got to checking the injectors while someone cranked the truck and they are only dripping fuel. A little you tube searching and found a video of the "proper spray pattern" and mine looks nothing like it.

My guess now is that because I used the sender and pump that came with the suburban fuel tank, my pump maybe weak. I'm going to swap in the pump from the saddle tank that was on the truck and see if it helps.

One question though, do the injectors only use the fuel pump pressure to spray or does it amp it up somehow?

 

[readymix]

There is a fuel pressure regulator built into the TBI unit. It regulates it, but it can't add pressure above what the fuel pump creates.

I would also check the filter.

 

[mckustomfab]

There is a fuel pressure regulator built into the TBI unit. It regulates it, but it can't add pressure above what the fuel pump creates.

I would also check the filter.

That's what I thought. So today I swapped out the fuel pump for one that worked fine with the saddle tank. I also put a new filter screen on it. This did not fix the fueling issue.

I went and bought a new filter and rented the fuel line pressure test kit. After I installed the filter I connected to tester between the hard line on the frame and the braided line that goes to the tb. Pressure while cranking was 0psi...

I thought maybe the switching valve was acting up so connected the tester right at the top of the fuel tank. Still getting 0psi...

So now it seems like my options are to buy a new pump/sender ($200.00 bucks) or, could it be that thefuel pressure regulator is letting all the pressure go back to the tank? I'm thinking about disconnecting the feed and return line from the tank, and putting some pressure down the feed line to see what comes out the return. Any options on this?

 

[readymix]

Is the fuel pump getting power?

 

[mckustomfab]

Yeah, I'm getting 12v at the in tank connection and you can hear the pump run. It just seems like it's not building any pressure. I also replaced the short piece of hose between the pump and the hard line on the sending unit.

 

[Russell]

Sounds like you have a bad pump. The impellers may be stripped out inside. Running but no pressure means either it is bad or it blew the hose off the top and it pumping fuel back into the tank.

 

[mckustomfab]

Update.

Well I think I have been fighting a combination of problems. I adjusted the air pressure on my compressor to about 25psi and disconnected the feed and return line. I tried to blow air through it but got nothing. So I disconnected the line on the fuel tank side of the switching valve and tried again. This time I got air, so I disconnected it before the fuel filter. This time I for no air, so I bypassed the switching valve and the truck stated.

It runs a little rough but I think its a combo of needing the base timing adjusted and a weak pump. My fuel gauge still reads at the 3 o'clock position but this may have to do with the switching valve too. I'm going to get a new switching valve and see what that does, I will probably get a new pump too but I want to see if the timing is what is causing the rough idle. Thanks for everyone's help and ideas. I will update in a few days when I get the valve installed.