I've learned a quite a bit about the Central Multiport Fuel Injection system this week. I've been having some drivability problems with my '94 Blazer (W) over the past couple weeks. The fuel mileage went down and it was running rough. It passed the fuel pressure and leakdown tests but I was still suspicious. I found a vacuum hose coming off the back of the manifold and that improved things somewhat. I still wasn't convinced.

I opened the plenum and found the left (FPR) side washed down and free of carbon which would normally indicate a leaking FPR. When I energized the pump I didn't see any evidence of the FPR leaking, however. Because none of the symptoms and test results made sense I took the injector spider out and took it to Dr. Injector in Tacoma for testing. He tested it and found a couple of interesting things: two of the nozzles didn't always open correctly; the nozzles were staying open too long and spraying too much fuel; the FPR was bad but would only leak when the system was at full pressure. Those results would explain why I didn't see a leak when the system was holding static pressure. It would also explain why the FPR side of the intake was washed down. Most importantly, it would explain why the Blazer was running rough.

In addition to replacing the spider I also replaced the nut kit. It's just a good idea to replace it when the intake is open. That part of the job proved to be incredibly difficult. I spent nine hours removing the bolt holding the fuel line bracket to the back of the manifold. Someone before me had tried to remove it with the wrong size of Torx bit and striped the head. I tried all manner of different bits and drivers to remove it. Because of the cramped space behind the plenum it was very difficult to get anything on that bolt. Eventually I got a fluted bolt remover on it and was able to get it out. Installation, to quote Chilton, "is the reverse of removal."

Now the Blazer has far more power than it did before. I did a WOT run over Graham Hill and it was amazing the difference in power. In the morning I have to check the brake switch. The computer is giving me an OBD I Code 38 which is a problem with the brake switch. The TCC isn't locking up as a result.

This was a fascinating and unusual fuel injector problem. I hope others will be able to use this information troubleshooting their trucks.

 

Very good write up.

While our current project is a 1998 that we hope to have running this weekend we did go with a new spider while we were redoing the gaskets. I wanted a new FPR too so when with the new MFI spider that has actual injectors at the end of each line since it was only $182 and a new FPR comes as part of the spider at no extra cost. Got it from Amazon.

Currently thinking age and mileage makes the spider an item to replace before it starts failing. It is hard on the rings and pistons and even bearings if they are getting washed down by gas.

 

Here's a postscript: today I was making a hard run up a hill and the #6 spark plug popped out of the head. I pulled over and put the spark plug back in the head. Make sure you tighten the spark plugs completely after you take them out to check their condition.

It was fortunate that I carry tools and gloves in the Blazer.

 

did yours act like it was flooded sometimes and then get a quick poof of smoke after start? but the smoke smelt like gas not oil so that would explain the wash down.

 

You could have saved a ton of headache by just removing the lower intake, buying the 10 dollar lower gasket kit, and removing the nut kit on the bench where you can get an eye on it at least. When those lower intake gaskets start to leak, you are going to see exactly what I mean.

Glad shes running good for you now.

 

I considered removing the lower intake manifold. I've replaced lower manifold gaskets on GM 3.4 and 3.8 V6es. It would have made a lot of sense to add the 4.3 to my repertoire but I wanted to get it back on the road quickly so I decided against changing the lower gasket. As it turned out, I spent so much time on that stupid bolt that it might have been quicker to just remove the lower manifold and replace the gasket.