I purchased a 1991 Formula 5.0 TPI that sat in a garage for the past 5 years. It ran great and when I got it home, I ran the gas tank down to an 1/8 then put synthetic fuel injection cleaner in and filled the tank with 93 octane gas.
Now the car will not run, it barely idles. I replaced the fuel filter, change the PCV valve and replaced the MAP sensor and still nothing. If I disconnect the first fuel injector on the drivers side closest to the air filter the car idles fine and when I hit the gas it does not stall. After some internet searching it says that the fuel injection cleaner is harmful to the stock Multec injectors.
Any suggestion on what can be wrong and why would the car idle fine with one injector disconnected?

Any feedback would be helpful....

Start with basic tests - ohm the injectors hot and cold and what is the fuel pressure. She's old and gonna break down now that she is put into service. If you are in it for the long run get the FSM, there are diagnostic charts that step you through what to do. Buy a fuel pressure gauge instead of parts that you aren't sure you need. Keep us up to date.

Talltim, I tried doing an ohm test but only on one injector and I think it's the bad one. I will try another one again tomorrow. I do have a fuel pressure gauge on the rail and it reads between 35 and 45 psi so I know that it's got fuel pressure.

Fuel injector cleaner is not always the answer especially in this case since the car sat so long. Using the cleaner more than likely broke away some of the crap that was in your tank and lines which ultimately may have gone into the injectors.
Multecs are junk anyway. You're better off just replacing them

Yep! Had lots of problems until I discovered the Multec were prone to failure after so many years. They actually tested ok on hot and cold ohm, so not a reliable test.


Best of luck!

Injectors are all full of crap now. Buy some new Bosch III's from SouthBay and be done with it. You won't regret it

Ok I ordered Bosch injectors from Southbay, will let u know how things go once I get them and put them in. Frank at Southbay was very helpful.
One question though, I still don't fully understand why when I pull one injector out the car runs fine, but when I plug that same injector back in and try pulling out another one the car will not idle.

If you have enough wire on the connectors, swap the Injector plugs on the first two. The injectors are Batch Fired, not individually. See if it does it when you disconnect the other one that you swapped the plug to.

Mine was doing the same craziness! I would pull one injector out and the car would go from rich too lean and vice versa. Depending on which injector I was unplugging, the BLM would react. The 02 sensor is on one side and reacts to 4 cylinders. I've seen some having the O2 sensor on one cylinder with headers! So when you pull one injector you create an imbalance in fuel metering and your ecm reacts. In your case it could be just a luck that when you pull the particular injectors it helps the engine. Remember, if your Multecs are toasted, the metering is all over the place.

I would not worry too much about that. Once you put brand new Bosh, the engine ran smooth like never before.


Best of luck!

Thanks for everyone's input

So after buying 19lbs Bosch D3 injectors from Southbay, they worked great. The car runs and idles much better. The job itself wasn't very difficult, just getting a few bolts out were a pain to get to, but other than that it all went smooth.

The Multec injectors fail by the coil shorting out. Once the ohms are low enough the ECM can't fire any of them. There is only one injector driver in the ECM for all eight injectors ('86 - '92 TPI).

It can take just one very shorted injector, or several injectors that are moderately shorted to cause issues.

RBob.

That's interesting. So if there is only one injector driver do we have an idea why GM put 2 separate banks of 10amp with separate grounds to ECM? I knew they were firing all at once but was always under the impression there were 2 drivers. Thanks in advance for your info

Just a guess, but if one side has a short to ground and the fuse for it blows, the engine may stay running long enough to get off to the side of the road.

RBob.

Make sense!

buy noid lights for the injector pulse ( or rent them from the parts store) and see if the injector pulse is fine or if it is shorted out. look at your harness and make sure no animals have chewed the harness and look at all your ground wires.