Tune & Log. Gen VI CBB 1999 Vortec 7400

[oldpartsnrust]

Ugh, I am so close I can taste it.... lol. So this weekend I did a running valve adjustment. But before doing that I noticed that there seemed to be a miss at idle. I narrowed it down to Cylinder #3. I did a spark test and a compression test. Unfortunately, my compression tester is not holding any pressure so the needle just bounces. However, it bounces up to like 120-125 and then back to 0. I didn't compare it but it seemed like that is what I recall to be a good number. I would expect that if it held pressure and did a bump up slightly with 2nd and 3rd pump that it would be about 130 possibly. I am working on getting my pressure tester fixed and will give it a go again tomorrow. The miss seems to go away under even slight throttle, so I am not thinking there is a compression issue. I also did the spark test and do see spark. I replaced the plug wire to test as well, no difference. HOWEVER, with this being a seemingly dead miss, until I bump the throttle, I am starting to suspect that this might be an injector. I plan to remove the plenum and swap the injector to verify, but am seeking opinions so I won't be totally in "light a match mode" if it doesn't work. I pulled the plug and it seemed completely dry. This is port fuel injection, so I am thinking that possibly there is not any spray from the one during the low demand at idle (or not enough) and that when bumping the throttle it is able to suck in some of the other cylinders' mix from the manifold, and/or the injector comes online enough to give some contribution to the power output? Does any of this sound plausible? This is starting to tax me now. I was patient so far, mainly due to the fact that most of my issues have been self inflicted, but this is worrying. Anyone have any input on my hypothesis? Any others seen this type of issue with the Vortec port fuel injection?

Thanks!

[Elm Street]

If you suspect an injector, I would swap it with another cylinder. Inspect lower injector O ring.

[abc]

I would start with being sure it is a miss. Does the B1 O2 sensor show the miss? You should see a specific sign wave in the MV trace. The trace should have a sharp drop unlike the B2S1 O2s. When you remove the plug wire while running, do you see the RPM change along with an IAC step change? Do you see the trace change on the log? Get a solid compression and leak down test? Pull the injector and watch the prime shot when you turn the key on, does it look like a known good cylinder?

[oldpartsnrust]

Rockers01.jpgRockers02.jpg

I guess I misjudged/misremembered when the miss came on... I did a compression check (now that I fixed my gauge, sort of... I only had the wrong schrader valve which can give inaccurate readings, but I figured the inaccuracy would be on all, so I would just compare) and got 75PSI on one and 125 on the one right next to it. Then I pulled the schrader valve out of the tester again and pushed 90 PSI into the cylinder and heard air leaking out of the intake. I then rotated the engine and it maintained leakage the whole time (except for when it was also coming out the exhaust pipe!). I then pulled the valve cover and backed off the adjustment and the leakage went away. SOOOOO apparently I had overtightened the valves a bit. I buttoned it back up with the, now loosened valves, and the miss is gone. WHEW... However, I am back to the rockers making a racket. I will get back on it this week and now that I KNOW FOR A FACT that the top end has plenty of oil, will do the normal valve adjustment and ship it! I put the little clippys on the rockers because last time I started it without the valve cover the oil shot over the fender! This time the clips kept it in check and it just ran along the head and then down the back of the head and I left about half a quart on the driveway by running it for like 5 minutes total!

[oldpartsnrust]

Alight, got back to this. I loosened all the valves, set it at TDC, set them all to 'no lash', rotated the crank 360 degrees, did the same. Then bumped it over like 8 or 10 times and verified each valve was at least at 'zero lash' and then spun them all a 1/2 turn more. Buttoned it up (this is after swapping out the roller tips) and there is some noise, but I guess that is how it is going to be... I verified the contact pattern on the valve tips, I verified there is oil galore getting up there and I guess it is the cam is just noisy... As long as I know the rest of the stuff is good, I am calling it done...

[oldpartsnrust]

So, I am very happy with the progress, and the support from everyone on the forum! I really would like to square up (in a positive manner!) with Sirius if anyone knows his contact information. I owe him bigtime. I really appreciate everyone's input, information, advice, support, encouragement, etc. and it helped me get this thing back on the road! I am fairly certain the remaining noises and such are simply a result of the valve ramps on the cam as there is no knock or hammering, but rather machinery noises. The next oil change should help refute or confirm that conclusion! I have no current indication of anything amiss as far as performance as this thing really exceeds my expectations, although I have not yet attempted towing anything. That is about 500 or 700 miles away yet. I will periodically post here with either good or BAD news! Thanks again!

[Maher Usman]

Man, I’ve been in the same spot—chasing a dead miss that only shows up at idle. Your injector theory definitely sounds plausible, especially with a dry plug. I’d go ahead with swapping injectors to see if the issue follows. It can be frustrating, but you’re on the right track!

[oldpartsnrust]

I got that figured, it was self inflicted (lol, again something I screwed up). I had overtightened the intake valve rocker and it was allowing compression to escape. I had tried the loosen till clack, then tighten till it stops, then 1/2 turn more... I think I just mistook normal, somewhat louder, valve train noise as clack. I think I am over the hump and now just finish the break-in and frequent early oil changes and am good to go.. Time will tell!

[Fast4.7]

Man, I?ve been in the same spot?chasing a dead miss that only shows up at idle. Your injector theory definitely sounds plausible, especially with a dry plug. I?d go ahead with swapping injectors to see if the issue follows. It can be frustrating, but you?re on the right track!

Fought idle and low load misfiring on 2 cylinders on my SBC 383 for a while. Eventually the culprit showed up via oil on the plugs. I had intake gaskets that got shoved out of position when the manifold was installed and had vacuum leaks on those two cylinder intake ports from inside of the lifter valley. Strangest misfire ever as it would go away under load but was worst at light part-throttle in lockup and overdrive at about 45-50 mph. Did not help that the cats that had been on the vehicle for 10 years were also coming apart and clogged.

[oldpartsnrust]

Figured I'd throw this in for fun. Here is when I was doing the frame.
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[GBull]

Figured I'd throw this in for fun. Here is when I was doing the frame.
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Nice!!