I think I have a leaking fuel injector. Rough idle/stumbling on a cold start in the morning. Strong smell of fuel under the hood at startup. After a half mile, idle is fine and it runs fine. Fuel mileage on the last tank went from 26 MPG to 22 MPG.

Appears that the injector is leaking down after I shut off the car for the night. Based on the mileage, it leaking as I drive as well.

Anyone ever have any luck resolving this issue with a fuel additive, or do you thing I need to pull the upper intake and find/replace the offending injector?

 

Could just be an o-ring and not the actual injector.

 

If I was only getting the gas smell, but the car was running ok, and the mileage was unchanged, I'd suspect an external leak - like an o-ring on the fuel rail or a leaking pressure regulator (I did previously have a leaking fuel press reg and it smelled of fuel when I drove.)

The rough idle and stumbling at start-up feels like a misfire due to a cylinder flooded with gas. If the cyl is full of raw fuel, that would also explain the smell.

Anyway - the injectors have new o-rings on them. About 1,000 miles ago, I had to replace the cylinder head gaskets do to an overheating issue. New o-rings were a part of the re-assembly. It ran fine up until yesterday. I'm not sure if this issue is related to the teardown and reassembly of the motor - I'm thinking not since the motor ran fine for the first 1,000 miles after I put it back together.

 

It could be a faulty o-ring, or one damaged on install. It could also be an injector that was not properly seated.

My step brother had a '96 Grand Prix with a 3100. He did LIM gaskets, did not change O-rings, had to rip it apart again and change them when they did not seal. Complained of performance issues but never let anyone help him with it. When we junked the car a year or two later, dad and I pulled his fuel rail, one injector was NOT seated.

If you have a bad seal, you will have a vac leak. The injector itself could just be failing.

As far as a fuel additive, if the injector is leaking, that's not going to solve the problem, it needs replaced. I've have a faulty injector diagnosed on my Grand Am by a guy with a Tech II scan tool (he shut off one injector at a time, looking for the one that produced the least amount of change in the car's idle, once found, that was the problem injector).

 

Well, I've read that a leaking Fuel Injector can be identified by simply pulling the fuel rail and injectors off the LIM. Then, by turning the key to the on position, but not starting the engine, the fuel pump will pressurize the system. Then just look for an injector that is leaking fuel. Prior to pulling the rail off, I'd also be able to check the o-rings at the rail and intake to determine if they are seated properly.

I'm going to run a few bottles of injector cleaner through it - although I also have doubts about that fixing the issue. I just dumped some "Pro-Gard with Techron Fuel Injector Cleaner" in the tank, but I just read on the internet that Pro-Gard only has half of the cleaning additive that the "Techron" branded bottles have, so I'll be buying the Techron for the next tank.

Anyway - still open to the opinions of others while I figure out what to do.

 

What you mentioned for checking the injectors I think would work, but you need to ensure you have something to catch the fuel PLUS, as I'm sure you know, getting the fuel raill off a 3100/3400 takes a while to do.

Have you checked for a leaking vacuum?? I had a vac leak on my 3100 (a line slipped off) and it was causing the fuel pump to work like CRAZY (after a short drive, the pump began HUMMMING LOUDLY and when I got home, I could smell gas). It was a quick/easy fix.

Quick vac check can be done misting a can of starting fluid on ANY area with a vac connection. If you have a leak, it will suck the mist in and the idle will change. Just a thought.

 

Per The Maniac - "I'm sure you know, getting the fuel raill off a 3100/3400 takes a while to do."

Hey, compared to replacing the cylinder head gaskets, which was only a month or so ago, pulling the upper intake manifold and fuel rail is a piece of cake.

 

I know the feeling. I did the head gaskets on my '94 Grand Am with a 3100 a couple years ago.

 

Figured I would re-open this thread to let you know that ChiChi appears to have been correct in his diagnosis of my engine misfire and fuel odor issues.

About two weeks ago, I pulled the upper intake off and pulled the fuel rail and injectors off - and saw that two of the injector o-rings were cut or not seated correctly.

So, I bought a good set of Fel-Pro o-rings, replaced all 12 (upper and lower) lubricated all the o-rings and their mating surface. I CAREFULLY seated all the injectors, buttoned up the top of the intake, and have had no issues since.

 

Sweet! Good to go...