AMC 360 missing on 3,5 and 2 - JeepForum.com
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post #1 of 45 Old 07-01-2020, 07:09 PM Thread Starter
Chrisinchwk
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AMC 360 missing on 3,5 and 2

Motor and mods,
79ish AMC 360, mild cam, honed and re-rung, valve's lapped, compression was 165-170. All done by the PO, no reason to doubt him.

Issue.

Within 20 seconds of start up it starts missing on 3,5,2. (Runns great for 20 seconds) Verified by pulling brand new plugs, all three are wet. Verified again with heat gun, same header tubes are only 170* others are 500*. Runs good above 1500-1800 rpm.

Issue started on a casual drive. Went pig rich coming to a stop and stalled. Limped it home. Next day ran fine, then it happened again, only for a few minutes. Now it is constant. Sounds like a slowly dieing electrical component maybe????


What I have tried

New plugs
Isolating fitech to confirm it's not the problem.
Switched plug wires around. Same cylinders miss.
New rotor. I run an adjustable for the timing control. Verified with a new one. Same cylinders miss.
Timing light on each wire. All show correct rpm when attached to the timing gun. Checked spark on all cylinders, all looked ok.
Ohm'd the blaster 2. 5.3 and .5 Spec is 4.7 and .7 Not sure if this could be the issue. Would be weird the missing cylinders never change.



Ignition. All almost new.

MSD 6AL digital 800 miles
MSD blaster 2 coil. Ohms at 5.2 / .5
Slightly out of spec? 800 miles
MSD 2 wire distributor,cap and adjustable rotor. 800 miles
MSD tech adapter very old
MSD 8.5 wires 800 miles. Ohm'd in spec.
Auto lite 65 plugs, brand new.

I have it running on a fitech fuel injection, with it controlling the timing. Initially I thought it may be that, so I hooked up the fitech to not running timng, it will just supply fuel. Nothing to do with ignition. Trigger comes from distributor.

It has been running excellent for a long time. So I did not see the need to confirm TDC, distributor timing (rotor is not lose). I could be low compression in those cylinders, which I will confirm tomorrow.

Any help would be appreciated.

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post #2 of 45 Old 07-01-2020, 07:39 PM
oldschool74cj5
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hello
have you checked to see if you have a bad intake vacuum leak? you need all three components to run. air, fuel and spark. sounds like you have run everything for the spark and some of the fuel. now i would concentrate on a vacuum issue.

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post #3 of 45 Old 07-01-2020, 07:47 PM Thread Starter
Chrisinchwk
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I sprayed all around my efi with carb cleaner, then just replaced the gasket anyway and sprayed again. Can't find any leaks. I did the intake gasket last year. Any other good way to check for a leak,?

Chris
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post #4 of 45 Old 07-01-2020, 08:27 PM
oldschool74cj5
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hello

i read your original post again. how long ago was the engine reworked? have you check the compression lately? the reason im asking is lapping valves doesnt work all that great. you actually want about 1/2 to 1 degree difference on the valve and seats. the reason is it helps clean the carbon and crap off the valve when it closes and also creates a better seal between the valve and seat. when you lap the valves now you have the same angle and possible for the carbon and garbage to cause the valves to not seal that well.
i use brake wash alot of times when looking for a vacuum leak because it doesnt burn well and i find it easier to see it choke out instead of a increase in rpm. you can make a redneck smoke machine with a solder iron and jar. then pump in the smoke and see where it come out.

oldschool
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post #5 of 45 Old 07-01-2020, 09:02 PM Thread Starter
Chrisinchwk
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The engine has maybe 2k miles on it since the work was done. I'm doing a compression check tomorrow.

Agree with your comment on lapping valves, motor was cheap, oil pressure and compression were excellent. Beggars can't be choosers.

When it was still carbed, I thought I may have an internal leak due to some oil consumption so I replaced the intake manifold gasket. It continues to use oil.

It was running really really good, until all of a sudden out of the blue missing on 3 cylinders at idle.

I'm hoping to exhaust every possibility external, before I dig inside.

Thanks for you ideas. I'm open to anything.

Chris
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post #6 of 45 Old 07-02-2020, 09:43 AM
Nor Cal CJ5
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Your wet plugs on 3 & 5 and 2 are a good clue and seem to be the only one your motor is giving you. Compression test will help with this to rule out vacuum leaks in the effected cylinders. Are the plugs wet with fuel or oil? If fuel, your fuel injection system is suspect. If oil, could be leaking through valve guides.

1972 CJ5 304 V8, 3 speed manual trans
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post #7 of 45 Old 07-02-2020, 10:39 AM
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Isnít FiTech a throttle body injection system? How could a wet intake system only cause a misfire in three cylinders. If the plugs have fuel on them then it sounds like there is an ignition problem.

Have you moved any of those plugs to different cylinders to see if the misfiring also moves with them? If could be the plugs are defective.
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post #8 of 45 Old 07-02-2020, 12:45 PM Thread Starter
Chrisinchwk
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The fuel injection is a TB, I think I have ruled it out as the problem.

Plugs are wet with gas, not oil. I put brand new plugs in it to rule out bad plugs.

I switched plug wires around, same cylinders miss.

I pulled all the plugs and confirmed spark on all of them, even the cylinders that are missing.

This afternoon I will be doing a compression check.

Any other ideas??

Chris
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post #9 of 45 Old 07-02-2020, 07:40 PM Thread Starter
Chrisinchwk
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Did a compression check tonight. All cylinders 150 +-10% except #3, big zero. Scoped the cylinder, can't see any damage.

Could this be the cause of a bad miss in other cylinders 5,2 if the if the intake valve is stuck open.

Anyway going to pull the covers tomorrow for a good look.

Thanks for the suggestions.

Chris
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post #10 of 45 Old 07-03-2020, 11:11 AM
Nor Cal CJ5
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Could be an issue from the intake heat riser cross over into the cylinder?

https://www.jeepforum.com/forum/f8/a...5-2-a-4402845/

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post #11 of 45 Old 07-03-2020, 11:16 AM
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Sorry - wrong link above. Working remotely on my phone😁

blob:https://www.jeepforum.com/5be75c92-4...7-b8cd938304e3

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post #12 of 45 Old 07-03-2020, 11:16 AM
chromegrille
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Broken valve spring(s) ?
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post #13 of 45 Old 07-03-2020, 11:22 AM
Nor Cal CJ5
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Or stuck open with carbon. Pull the valve cover first and see what you find. Then you may have to go deeper - intake
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post #14 of 45 Old 07-03-2020, 12:24 PM Thread Starter
Chrisinchwk
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I will be pulling the valve cover tonight to see what's going on. Considering how it manifested, on and off, I'm thinking a valve or two stuck open. Question is the what.

My crossover was blocked off by the PO. Not sure how this could cause zero compression in one cylinder, unless the intake manifold cracked. Even then I would think there would be some compression.

When I say zero compression, the needle doesn't even vibrate when it's turned over.

Chris
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post #15 of 45 Old 07-03-2020, 12:26 PM Thread Starter
Chrisinchwk
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Quote:
Originally Posted by chromegrille View Post
Broken valve spring(s) ?
I know the valve didn't fall into the motor, scope confirmed no damage to piston or cylinder wall.

Tonight the valve cover comes off.

Thanks!!

Chris
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