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Aug 9, 2016 | 11:47 AM

jbarendrecht85

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Any advice please.

OK so about a week ago I came home and went back out to start my 77 -f-250 7.5L I was idling really rough so I figured maybe my plug or something got fauled up and when I checked to see what cylinder was not firing it was my number 1 cylinder so this is what I have done so far put new spark plugs in new wires new cap and rotor, tested cylinder compression and all of them tested all the same give a psi or so on each one, but that cylinder is still not working. No I don't really know what to do next. Could the coil cause only one cylinder not to fire or could the timing be off, I really can't see that just affecting one cylinder but I don't know maybe it could? Please if anyone has some advice on what to try next would be greatly appreciated.

Aug 9, 2016 | 02:08 PM

Filthy Beast

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From: Taxachusetts

Please use proper sentence structure and punctuation and avoid run-on sentences, ok? It helps while reading/trying to figure out your diagnosis.

With that said, no, the coil would not cause a no-fire situation on only one cylinder, nor the timing.

The firing order/plug wires have been checked and are correct, yes?

Have you heard any "ticking" sound from that side? No carb backfire?

Pull the valve cover on the naughty bank, fire her up and watch the valves on that cylinder compared to the ones next to it.

Get back to us with your observations, please.

Aug 9, 2016 | 02:24 PM

dlburch

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if the compression test was good on that cylinder, I wouldn't suspect any valve train issues. what intake are you running? Any vacuum leaks that would cause that cylinder to be lean? One longshot, did you inspect the reluctor in the distributor to make sure all 8 teeth were ok, clearance was good?
Also don't eliminate the possibility of new parts being bad...I know, you would have to be really "lucky" to have another new bad part on cylinder 1, but it could happen.

Aug 9, 2016 | 03:41 PM

jbarendrecht85

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Sorry was in a rush when I posted this problem. Yes, firing order is correct. Can't hear any ticking or any noise coming from the cylinder. Pulled the valve cover off and all of them are moving the same when running. Yes it does backfire, it's when accelerating fast from idle it will backfire through the carb. The plug has a good spark, would that still happen if the distributor was bad? How would I test for a vac leak? I shot some starter fluid along the top where the manifold meets the head and there was no change in idle. Now I'm under the hood racking my brain trying to figure out why this cylinder is not firing.

Aug 10, 2016 | 08:19 AM

Filthy Beast

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From: Taxachusetts

Quote:

Originally Posted by dlburch

if the compression test was good on that cylinder, I wouldn't suspect any valve train issues.

I was thinking along the lines of a cam lobe going flat, a shot lifter or a broken valve spring. He'd still have compression but the backfiring through the carb deal makes me think cam/lifter problems.

Quote:

Originally Posted by dlburch

One longshot, did you inspect the reluctor in the distributor to make sure all 8 teeth were ok, clearance was good?

You might be on to something here....OP; do what dlburch said - the clearance between reluctor and stator should be ~ 0.018". Also check the reluctor for any excess movement - sometimes the roll pin holding it to the dist shaft shears.

OP - Pull the offending plug - what does it tell you?

Hook up a vacuum gauge to manifold vacuum, start the engine and observe the readings. What does the needle do and how much vacuum in inches Hg do you have at idle?

Aug 11, 2016 | 04:11 AM

Ozzie H.

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From: S.E. Louisiana;so far USA

You said "when I checked to see what cylinder was not firing it was my number 1 cylinder". I'm assuming that meant that number 1 was completely dead and not just weak. I'm also assuming that all the others were normal. If the valve motion and compression was normal for that cylinder then the internal aspects are probably O.K. The new replacement parts are probably good, but it cannot be assumed. Verify the performance of each part somehow. The others have provided some good ideas to check out. After you have checked them thoroughly, please report back with your findings. It appears that the problem is only affecting #1 and is not an easy one to find.
While you have the #1 spark plug out for inspection you could connect it to the spark plug wire, lay it against a ground on the engine and see if it fires normally.
I look forward to hearing the solution of your problem.

Aug 11, 2016 | 10:24 AM

HoustonDave

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From: NE Texas

You need good compression, gas, and spark for the cylinder to fire. Compression and visible valve function are good. Assuming you don't have a plugged manifold, all the rest of the cylinders running indicates fuel is not an issue. Your most likely issue is electrical - first thing I would suggest is swapping your #1 plug wire to another cylinder and see if the miss migrates with it. Do the same to the plug. Carefully check the distributor cap and make sure you have continuity from the underside to the top side. Check the resistance in that plug wire? Also try swapping in a brand new plug - it is not unknown for a plug to fire fine at atmospheric pressure and fail to fire under compression (I have personally witnessed that.) If all these fail - I would suspect your ignition box or distributor but it would be very odd to have one particular cylinder fail to fire like that. Just my $.02.