TBI still missing, failed emissions, code 34...

What about spraying carb or brake parts cleaner to look for a potential vacuum leak?

I tried that a while ago to no effect, but it sure seems like the intake gaskets are the culprit, there isn't much else that it could be. Just for giggles, ill try that again though.

edit; still no effect

rob249 said:

Still trying to get my 94' Yukon to stop missing at idle... ...I failed emissions, high HC, but the CO is quite low.

Click to expand...

rob249 said:

I still have a rough idle, probably wont pass emissions still.

Click to expand...

Low CO indicates that the cylinders that run, have a reasonable fuel/air ratio.

High HC is usually due to misfire.

In my driveway, that thing would get a cylinder-balance test to see WHICH cylinder(s) are misfiring. OBD2 would have a sensor and computer programming for that, but with TBI / OBD1, you have to figure out which cylinder(s) are weak/dead the hard way. Well, maybe you could check header-tube temperatures with an infrared thermometer looking for a "cold" tube. Harder with iron manifolds.


Connect a vacuum gauge to manifold vacuum. You could also use a tachometer, but the vacuum gauge works better/more sensitive.

Blunt the sharp points of eight small "finishing nails". Lube 'em with Silicone dielectric grease, and SLIDE them between the plug wires and the distributor boots of each plug wire assembly. You'll feel when the nail touches the metal terminal of the plug wire. DO NOT puncture the insulation of the plug wire--just slide it between the insulation of the wire, and the inside of the distributor boot.



Then start the engine, and touch each nail in turn with a grounded jumper wire, or grounded test light. (The test light won't light up.) Short the spark to each cylinder in turn, for not more than 5 seconds. Then let the engine recover for five seconds before moving to the next one. Watch the vacuum gauge. Cylinders that don't drop the vacuum as much as others are weak. Cylinders that don't drop the vacuum at all are dead. The more the vacuum drops and the RPM decreases when you short the spark, the more healthy that cylinder is.

That narrows down the misfiring cylinders--they won't drop the vacuum much or at all. And that, in turn, helps with the diagnosis.

Coming from someone who had a faint miss at low idle at WOT, would not hurt to check your compression.

Schurkey, that is a genius way to find a bad cylinder. I've been looking for my compression gauge, i have one in a box somewhere, but that will work.

Bottom line here is that I just need this to pass emissions for now, my other truck needs work badly(Ford), but I cant take it apart until I have something else to drive. Wondering if I can just lean it way out with the idle screw on the TBI, but I think the IAC would just compensate and make that pointless.

edit; it should be noted that all the plugs look good, none are fouled or wet, etc.

Pay attention to the vacuum gauge when performing the cylinder-balance test.

Then you don't need to compression-test more than one "good" cylinder; along with any "bad" cylinder(s). You wouldn't have to test "all" the cylinders. Could be a time- and effort-saver.

(OTOH, it's not a bad idea to test 'em all as a baseline, then if you have issues later you have something to compare to.)

Sorry everyone, I was down with the sickness last week. Now I have my ignition system setup with the finishing nails, and I have my Vac gauge ready. Just wondering what vac port I should hook up to on the TBI. It appears the TBI unit has a left, right and ported. Do i need to switch left and right depending on which side im testing? Would a ported vac work for both sides, im guessing that a ported line wouldn't matter since im just looking for a difference?

edit; What should I ground the wire to? Is an existing ground bolt ok, or would it be a bad idea to use a ground bolt that the is already being used by the TBI?

So, I just did the finishing nail procedure, Im getting a vacuum drop on all cylinders when i ground the plug wire. I get slightly less vacuum drop on cylinder 8, all other seem too have the same drop. Pic is baseline vacuum before grounding the plug wires.

Apparently, #8 is your problem child. Look at the intake runner for #8. Any vacuum ports? Could there be a vacuum leak into that runner?

Problems with the #8 terminal on the distributor cap? Failing #8 plug wire? #8 spark plug?

What is the cranking compression of #8 compared to a "good" cylinder?

Pop off the valve cover, make sure the rocker arms for #8 move as much as the rockers for #6, #4, and #2.

Checked cap, and rotor, they look good, just to be sure i cleaned them. Plug is a little sooty, but decent looking to me(ill devert to the experts on this), gap at .035. Wires are Accel 8mm with ceramic boots, plug is also Accel, all about 5 months old only 4000 miles or so. Going to look in the shed for my compression tester, and grab some valve cover gaskets at the store. Also need to do some engine cleaning before i take that cover off and before i can get a good look at that part of the intake. Sorry about the bad focus on the plug.