two cylinders running lean, misfire, please help???! - Hot Rod Forum : Hotrodders Bulletin Board
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Old 12-21-2011, 03:45 PM
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two cylinders running lean, misfire, please help???!

HI everyone,

I have a problem and I'm tearing my hair out. I recently bought a new car with a carbed L31 vortec 5.7.

I noticed it ran rough at idle, seemed to be missing a little, revs dropping down, bit of a shudder. With help on a previous post I have checked timing isn't fluctuatiing at idle, and there doesn't seem to be a vacuum leak. I changed plugs, leads and ran some rislone in it (it had a noisy lifter which has since quietened down).

None of there things seem to make a difference, so I gave up and took to mechanic, who richened up the idle mixtures till the plugs fouled, checked the timing (32 in total, 20 mech advance and 12 vac advance) crashed the car, and said the next step is to pull the motor apart as it wasn't anything obvious.

So I pulled all the plugs again, and noticed the plugs in cylinders 5 and 8 are running very white, whereas the others all have a bit of color. So (probably in ignorance) I am thinking lean misfire on those two cylinders. None of the plugs are fouling now as I reset the mixtures with vac gauge.

So ran a compression test this morning and these are the results for each cylinder in PSI (cold motor cos otherwise i can't get in around the headers, throttle open, all plugs out)

1. 220
2. 225
3. 220
4. 215
5. 225
6. 215
7. 230
8. 225.

Right, in general I understand the variance should not be more than 10% between cylinders. I make the variation between 230 and 215 (highest and lowest) to be around 6.5%. Are these normal readings? Otherwise they seem much higher than what my Vizzard book (How to built your sbc) says they should be?

Anyway, back to the problem. I have two cylinders running what appears to be lean. Any suggestions what this may be? I am thinking maybe valves not adjusted right, maybe a failed lifter or something.

Would a lifter not opening properly, or not closing properly cause a cylinder to run lean?

Any other suggestions as to what might be causing this if it can't be valves or lifters? I am hoping if I can solve the 2 lean cylinders, it may also solve the rough idle problem.

Please any help, directions, advice would be greatly appreciated. I don't want to go back to mechanics after they crashed the car last time, and really want to drive it for our sunny christmas!

Thanks guys really appreciate your time.

Oh, not sure if its related, the chap before me was running 10-40 oil, I changed to synthetic 5-30 as thats what 97ish tahoes and suburbans recommend. Could thinner oil be causing lifter to not pump up enough or something? I would be very happy if simply changing oil could fix this :-)

Cheers

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Old 12-21-2011, 05:25 PM
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Kiss Keep It Simple...

Keep This Simple.compression Is Super. Yeah Check Your Valve Ajustment. Timing Sounds Good,it Wouldn't Affected Just 2 Cylinders. Run It For Awhile If You Think Lifter Is Sticking
But Back To The K.i.s.s. Program... I Think You Have Vacuum Leaks On
Those Two Cylinders, Just A Gut Feeling. Dave
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Old 12-21-2011, 05:42 PM
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Hi Dave

Thanks for your reply. I thought it was a vac leak too, I checked it with visually, with a hose to the ear, and wwith propane, and can't seem to find anything. Vacuum gauge shows around 20" at idle with a rapid fluctuation of less than an inch. The mechanic said he had ruled out a vacuum leak too, which is why I am looking at other options.

Is it possible it could be an internal vacuum leak, maybe into the valley where we can't find it? However my research indicates if thats the case, it would be sucking in oil and fouling the plug, which it isn't doing??

Anyway, thanks for your reply I will try checking the adjustments on those to rockers and see whats what.

Cheers

rob
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Old 12-21-2011, 10:32 PM
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Make sure the engine is grounded to the body
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Old 12-22-2011, 10:50 AM
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Okay, I just went out and looked at my SBC . #5 and #8 share the same intake runner, which in the case of many engines, has the port for the power brakes. You may have a bad brake booster or a hole in the brake booster hose. Take the hose off of the intake and plug the port on the intake to see if that is the problem. Let us know. Allan
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Old 12-22-2011, 01:53 PM
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Allan,

Your a genius. I never made the connection between the two intake runners. Someone earlier suggested to keep it simple and that it was likely vacuum leak.

I went through the process of checking for leaks about two weeks ago. Massive hole in brake booster hose, so replaced that. Tested the brake booster every which way, seems to hold vacuum etc, so I eventually discounted that. The trans modulator and brake booster both are tee'd off that runner.

So, although I can't find a leak, I think you are right on the money, the coincedence is too great, so I will stop playing with my valves before I break something, and try running without a booster today and see what happens. Many thanks

and azduffee, thanks for the suggestion, are you able to give me some theory behind it?

Thanks guys fingers crossed

Rob
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Old 12-22-2011, 02:21 PM
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Quote:
Originally Posted by holden350
Allan,

Your a genius. I never made the connection between the two intake runners. Someone earlier suggested to keep it simple and that it was likely vacuum leak.

I went through the process of checking for leaks about two weeks ago. Massive hole in brake booster hose, so replaced that. Tested the brake booster every which way, seems to hold vacuum etc, so I eventually discounted that. The trans modulator and brake booster both are tee'd off that runner.

So, although I can't find a leak, I think you are right on the money, the coincedence is too great, so I will stop playing with my valves before I break something, and try running without a booster today and see what happens. Many thanks

and azduffee, thanks for the suggestion, are you able to give me some theory behind it?

Thanks guys fingers crossed

Rob
It could also be a hole in the trans modulator diaphragm or the line to it. I'd plug off the port at the intake and see if the vacuum stabilizes and go from there. You can drive with the modulator line plugged. Just shift manually to avoid early upshifts.
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Old 12-22-2011, 04:44 PM
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White plugs can also mean that cylinder is not firing at all. The raw gas will keep them very clean. Also on the intake if they are sharing a runner it could be sucking in a little coolant which will also make it run rough. If this is the case you should see the coolant going away.
I would put a spark tester on the cylinders in question to make sure they are firing before you dig in too deep.
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Old 12-23-2011, 06:10 PM
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I just fired up a 428 in a 68 Shelby and 4 of the exhaust ports were not warming up. Prior to that, I noticed the ground strap was not tight on the firewall so I tightened it and the problem completely went away. It purrs. Probably the easiest fix I ever had.
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