91 4.3 v6 new fuel injectors don't work

I have a 91 s10 blazer with the 4.3 liter engine with TBI fuel injection. One of the two fuel injectors was shorting out after it would get hot (heat soak). I determined this by checking the resistance across the terminals. It was 3-4 ohms cold and it would short when heated. The other one was OK. It would run fine while cold but if you let it heat up and then shut it off to heat soak for 5 min then when you started it up only one injector was working. I purchased two BWD replacement injectors and put them in and it would start ok then stall out about 3 seconds after running. You could not put the car into gear or keep it running. I took them back and ordered AC delco units and put them in then it ran better but still not correct. I would surge up and down during driving like they were not metering the gas correctly. I put the old injectors back in and it runs fine until it heats up (just like it did originally). I've checked all of the sensors and the fuel pressure and they are all good.

Are you using a new TBI rebuild kit with the new ACD injectors? If not, I'd recommend replacing all of the gaskets and o-rings, as well as the fuel pressure regulator diaphragm. The pressure needed for the old injectors may not be enough for the new ones and an old, stiff diaphragm may not allow enough pressure. You might even consider converting it to an adjustable regulator (wish I still had my '92 for this!). I recently did it for my C2500 and did notice a power increase.

The TBI is simple to work on but can be finicky...it doesn't take much to make it run poorly. Also check the emissions control tubing and rubber connectors for cracks or loose fit which the surging may indicate a leak somewhere.

Good luck!

jdmccright,
I did not rebuild the TBI unit but I did replace the Pressure regulator diaphragm and spring. I checked the fuel pressure with at the fuel filter and its a steady 12 lbs. I have checked the hoses for leaks, most are new and in good shape. I would think if it were a vacuum leak then it would not run well with the original injectors also. I even removed the EGR valve and cleaned and inspected it and it is fine. Would the new injectors need more than 12 psi?

GM spec fuel pressure is 9-13 psi, but most are set at the factory between 11-13, like yours. The new ones should run better frankly, but I think the surging is a symptom of another problem. Vacuum leaks can be easily found by spraying carb cleaner at all the hose connections and around the TB base...if the engine stumbles, you found a leak.

I would also test the IAC valve (don't try to compress the plunger by hand) and clean the pintle and plunger seat, and also test the MAP and ECT sensors and their connectors.

jdmccright,
That all makes sense but the thing I don't get is that when I put the old ones back in it runs just fine. I would think if it were a problem with the vacuum it would not go away, same with the sensors. I have an 89 s10 pickup with same motor and I swapped the IAC from it, no difference. I've put the old ones back in twice now just to be sure and both times it ran just fine so I'm inclined to believe the problem lies in the fuel injectors themselves or there is some sort of Computer learning procedure that needs to be done but the install instructions don't say anything about it. The ACD ones run better/different than the BWD units I tried. I couldn't even get the car to run in gear with the BWD units and with the ACD units it will drive around its just erratic. The ACD units will idle OK.

Have you swapped the original injectors left/right to see if it is one particular injector that is failing? Also, did you unplug the injector before measuring the resistance? Sorry for the grinding questions, but it seems at this point the devil is in the details.

If you're convinced the problem is in fact one injector, have you exchanged the bad injector with an ACD unit?

Also, have you compared the spray patterns? You can actually use a strobe timing light to see the spray cone or if the injector is dripping.

jdmccright,
I tested the resistance across the terminals while the plug was off. Both old injectors read 3 ohms while cold. If you heat them up with a hair dryer one still reads 3 ohms and the other one shorts out. The one that shorts out does not spray any fuel after it shorts. IE if I start the car in the morning (cold) it runs just fine. Then if I run it until its good and hot then shut it off and let it heat soak for about 5 min then when I get back in and start it it barely runs and when I look at the injectors the bad one is not spraying any fuel. That's how I know that one of the injectors is bad. Once I figured all that out I figured it would be a simple job to just replace them both but so far it's not worked out that way. I have not switched sides with them but I did flip the plugs around initially with the same result. I have not tried to replace just the bad injector. I have not tried to compare the spray patterns with a strobe but I would say that the original injectors look like they spay a finer mist than the new ones.

Sounds good. The thorough answers help alot and just confirm that you've done due diligence in narrowing it to the injector.

All I can think of at this point is to check the sensors/valves mentioned before and also see if the two new injectors are working at similar flow rates. I believe yours are matching injectors but the first TBI units (1987?)had one low and one high flow rate injector to improve fuel economy. They are color coded on the center and should be the same color for you...yellow/blue?

jdmccright,
I checked the ETC and the MAP sensors and they are both good. The injectors are both the same blue/yellow. Also I just remembered that the last time I put the old ones back in I did swap them around and there is no difference it still runs fine while cold. I have not done the strobe light check yet.

Just for ha-ha's, what is the resistance of the new injectors?

Tech II,
3 ohms, same as the originals...

I ended up sending the old injectors to Linder Tech in Indiana. The one was bad so they replaced it with a core they had in stock (old original). Then they cleaned, tested and flow matched both of them. I recd them today and put them in. The Blazer now runs perfectly. After 3 sets of new injectors that were supposed to be correct for the application..... It seems to me the problem is that there has been some consolidation of the after market injectors with the theory that they were "close enough" but apparently close enough doesn't cut it, not in this vehicle anyway... Thanks to everyone who posted advise above and I hope that this can help someone in the future to avoid hassle I went through. I certainly did learn alot about how the TBI system works and how to test all of the sensors though. :)

Watch for bad copper wire + connectors, see www.automotiveforums.com/vbulletin/showthread.php?t=1080822