Another "missing" thread

[rgfllamas]

I've spent a good part of the afternoon reading about solutions to erratic running LT5's. I'm no closer to understanding mine than when I started, hopefully the brotherhood can shed some light on my situation.

Car is a stock 1991 which I've had for about 7 years, 32K miles. It sat for about 6 months last summer (half tank of fuel with Sta-bil) and when I got back to Florida found it had a slight miss. Took it up to Sunoco and topped it up with hi-test. No change. About 2 weeks ago went to Cars and Coffee show, about an hour south of me on I-95, going it ran OK. Instant fuel economy showed about 27-28mpg running at 70-75 on the highway. Spent about 3 hours at the show and when I started the trip home it ran like crap. On the same highway I'm now getting 19 or 20 mpg, no power and shaking pretty bad at idle.

Got home and grabbed the inductive pickup timing light and checked each plug wire, all showed spark. Parked the car.

Time for new plugs. Put in a set of AC Delco 41-602 gapped at .042. Each of the plugs coming out were quite sooty and looked "rich" all uniform, however.

The new plugs helped but it's still not right, there's still a miss. I hooked up the fuel pressure gauge, key on engine off it jumps up to 48lbs and holds. I let it sit for 5+ minutes and had no leakdown. I then checked resistance across the coils and found all four to be between 15k and 17k ohms.

With the old plugs being uniform I'm thinking the problem isn't injectors, I think they're the originals, if one or more were not metering correctly would it not show as being lean on that particular cylinder? No leakdown of fuel pressure again makes me think it's not a leaking injector.

Ignition, if it were a failing coil or bad plug wire, would it not show up on one or two plugs? If one or two cylinders aren't firing properly won't the O2 sensor see the rich condition and try to compensate by leaning the mix going through the other cylinders?

both O2 sensors? plugs from both banks appear rich?

I know my reasoning is flawed but I'm not sure where or why. If spark, fuel and air were all correct I wouldn't have any problem. I don't have data logging capabilities and I don't want to start throwing parts at it.

[Demps]

Sounds similar to an injector issue.

More to it than my keyboard-warrioring but if they are original your description is a common lead-up & symptom.

Ted

[Paul Workman]

Sounds similar to an injector issue.

More to it than my keyboard-warrioring but if they are original your description is a common lead-up & symptom.

Ted

Yes... X 3 for me!

And, just to add, an injector going south is insidious. I attribute a burn valve to such an injector gone bad.

Once the motor is up to operating temperature, measure the resistance across each injector. Then should all be within ± an ohm across the lot; approx 14 ohms, give or take depending on the type installed. However, if you find one that is significantly lower than the rest, e.g., 9-10 ohms when they should be (say) 12 or 14 or whatever, you've got one going "south". And, if they get down to 8 ohms or less, they're gone and rough idle and missing are the order of the day.

Tell Jon at FIC that you have an LT5 and give him the mo. year and he can fix you right up with NEW STAINLESS injector sets and the Viton (sp?) rubber seals from Jerry's Gaskets (16...replace them ALL!). No more alcohol vs. injector problems.

[rgfllamas]

No doubt it's time for my initiation, first plenum pull. I can't complain, doubt if few have gone so long without this right of passage.

I'm still confused and concerned about the sudden cause of rich running. All eight plugs were identical, all sooty and obviously rich running. Even the exhaust tips blackened. I'm sure whatever caused it was sudden and happened all at once. The car has not been doing this prior to this outing. Could a load of "bad" gas cause a mixture shift?

[Paul Workman]

No doubt it's time for my initiation, first plenum pull. I can't complain, doubt if few have gone so long without this right of passage.

I'm still confused and concerned about the sudden cause of rich running. All eight plugs were identical, all sooty and obviously rich running. Even the exhaust tips blackened. I'm sure whatever caused it was sudden and happened all at once. The car has not been doing this prior to this outing. Could a load of "bad" gas cause a mixture shift?

Well, with this additional information, I'd say you're right for being confused.

Individual injectors destroyed by alcohol fail to open correctly, thus starving that respective cylinder of fuel: i.e., run lean because of failure. But, carbon/dirty black and/or wet plugs on the entire bank of plugs...is indicative of something else common to the entire set of plugs, e.g., O2 sensor not working.

I'm NOT saying that injectors are not also in need of replacement, but scanning the motor will eliminate a lot of guesswork by providing a wealth of comprehensive data - thus eliminating a lot of guesswork (and expense of changing parts that were not failing...). Do the easy thing, and change the plugs. Then, let's see what a scan turns up before "shotgunning" this problem further...is my suggestion.

[rgfllamas]

Both banks at the same time, and the plugs are identical, absolutely no difference side to side.