Rough Idle

[TeaBag]

I have been chasing a intermittent rough idle on my 91 GW which has developed into a constant and more pronounced rough idle; what I thought was a misfire. The entire car shutters when it occurs. It has an AFI TBI kit on it and a TFI upgrade to the distributor. It runs great at all other conditions except idle. Smooth, acceleration, great pick up. Plenty of power. The engine has 70K miles on it. Oil pressure is excellent. The engine is quite and makes no unusual noises.

Data from ALDL shows nothing out of the ordinary.

I have:
Replaced all plugs (previous were very clean and dry with NO indication of a fouled plug)
Replaced all wires
New cap, rotor and coil with TFI upgrade plugs gapped for TFI
Plugged all vacuum ports and done a smoke check with good results
Compression Test results
1. 170 2. 170 3. 170 4. 170 5. 185 6. 170 7.160 8. 160
Vacuum test at idle indicates 18" vacuum however, needle has about 1" jump when the "misfire" occurs. Other wise it's behavior is normal.

Any thoughts on where to go next?

[Nikkormat]

Hmmmmm with fuel injection your speaking a different dialect of AMC than I'm used to. But, in trouble shooting a less severe miss on a friends 4.3 TBI Chev that couldn't be diagnosed in iac, injection duty cycle, or any other usual culprits. The o2 reading would dip and stay consistently low, checking the fuel pressure also revealed that as soon as the motor went into open loop operation there was an unusual drop in fuel pressure. We tried everything we could think of. Later we were watching it idle sans air cleaner and noticed that the fuel mist from the drivers side injector looked awfully more potent than the passenger side. Sure enough we replaced the injector and problem solved. The seal on the very tip of the injector where it seals in between pulses was split and the plenum vacuum was sucking fuel past. So grab a flashlight and look for abnormalities.

[Nikkormat]

What about your vacuum advance system? Is it hooked up the same way as stock? If so you have different vacuum advance when cold then you do warm. So run a leak test cold and warm.

[TeaBag]

No Vacuum advance. Dizzy is locked. Spark advance is controlled by the ECM.

[Tatsadasayago]

Going off the info you provided I would suggest you have a vacuum leak. At idle is where manifold vacuum is the highest and this will aggravate the effects of a leak whereas at higher RPM the leak is overcome by air/fuel velocity. I would suggest you start at the base of the TBI and check each gasket surface between there and the intake ports. You can use a spray bottle of water or can of BrakeKleen and hit each gasket area listening for a change in the idle as you spray. If that doesn't yield anything, I would suggest a thorough check of all vacuum lines...including any under-dash routing. 91s have a vacuum canister that was prone to cracking as well as a problematic vapor recovery canister. Check these for cracks and for cracked, dried out or oil swelled vacuum hoses at the canisters.

Placing a vacuum gauge on the engine will help you greatly while troubleshooting.

Good luck!

[TeaBag]

Vacuum test at idle indicates 18" vacuum however, needle has about 1" jump when the "misfire" occurs. Other wise it's behavior is normal.

Plugged all vacuum ports and did a smoke check with no vacuum leaks detected. All vacuum lines have been replaced.

I pulled the air cleaner and watched the spray pattern of the injectors and both look normal and symmetrical. Also, WINALDL data show normal mixture and O2 data from idle through highway speeds. IAC counts are normal.

[letank]

sticking valve(s), or sticky hydraulic lifter(s).

Marvel Mystery OIl, transmission oil (dextron or whatever we put in our trannies) can sometimes free the issue.

[Tatsadasayago]

The jumping/fluctuating vacuum gauge needle does indicate a problem in the valvetrain...

[TeaBag]

Any thought as to further diagnosing without pulling the heads off?

After further thought and research I'm wondering if it is a potential spun Lobe. Compression on all Cyl is high so I don't think a valve is sticking open but it may not be opening at the right time? Or possibly a Valve that is stuck closed?

Ever heard of that happening on the AMC 360? Engine only has 70K original miles and with the exception of the TBI kit, is all stock.

One more thing to add, the exhaust does not sound right. Not sure how to explain it.

Thoughts?

[Tatsadasayago]

I'm not sure what you mean by 'Spun Lobe' . I can only assume you meant that the camshaft lobe has been flattened. The is no physical way for a camshaft lobe to rotate without breaking the shaft at each side so that's certainly not your issue.
If the cam lobe has been flattened on the intake side of a cylinder you would hear a funky 'waadadadadawap' if you stab the throttle to wide open briefly. If the exhaust lobe is flattened you won't hear much in the exhaust except for a throbbing sound ever engine revolution.
You can visualize the exhaust pulses by holding a dollar bill (Like a curtain) at the tailpipe. If a cylinder misfires, the dollar will get sucked into the tailpipe briefly. You can compare the frequency of the exhaust sound/miss at idle with the vacuum gauge. An ignition related misfire will often have little effect on the vac gauge needle whereas an intake valve issue will cause a steady drop of around 1" vacuum.
You can Do a bit of troubleshooting by removing the valve covers, one at a time and check the valve train visually then while idling. If you have a beat up rocker bridge or rocker arm you should be able to spot it by wiggling the rocker when both valves are closed. You can remove the rockers and bridges to check for valves pulling through the retainers, weak or broken valve springs and even check for loose valve guides. Lots of things to check for the low price of a set of gaskets and some effort.
By the way, if you can record the sound of the engine when it's missing and the squeak occurs as well as the exhaust sound...I bet you there are several members here who could zero in on the problem for you. A good video recording with good sound can be indispensable!

Lastly, I will say that in the era the 360 was designed engines were often worn out and dead by 60k. People forget this and are spoiled by modern vehicles whose spark plugs aren't scheduled to be changed until after 100k. So yeah, 70k miles of fair/moderate use is exceptional for your engine.

[TeaBag]

Thanks for all the advice. I plan do dig deeper into it this weekend. Pulling valve covers and so on.

With respect to the Spun Lobe comment. My understanding is some cams the lobes are pressed on and some are cast in place. For cams with pressed lobes, they can actually spin on the shaft and cause valve timing to be off with otherwise good valves, rods, tappets and rockers. Also, you wouldnt see it on a compression test. Obviously, cams with cast lobes would not have this problem. Not sure what type of cam this engine has.

Evaluating my exhaust, it DOES suck air in during the misfire.

I'll see if I can post some video this weekend.

[Tatsadasayago]

I would bet Stuka/Brandon's life that your camshaft is cast. (Waits for the ban hammer)
I cannot say with absolute certainty, but if someone has made a pressed-lobe camshaft for an AMC V-8 I have never heard of it.

When you get a rocker cover off, connect the plug wires and give a good look n listen as the engine idles. If you have one valve not opening very far (Lifter bleeding off to much oil and collapsing at low RPM and oil pressure) you should be able to spot it with your eyes or locate with your ear.

Good luck and let us know what you discover.

Jim

[TeaBag]

Having no luck. Pulled both rocker covers and watched the engine idle. Every thing looks and sounds normal. The car runs great (no misfire) at every speed above idle. Back to the drawing board.

[Tatsadasayago]

Hmmmm

It may sound odd, but have you checked the distributor shaft for side play? I remember having a similar problem with a 340 mopar with Elect Ignition that missed intermittently at idle only. Turned out the dist shaft bushings were loose enough to cause the reluctor to move just far away enough from the pickup to cause a non-fire event.
Besides the physical 'wiggle check' putting the engine on a smart scope sure would reveal alot about the firing patterns...and any decent tech would be able to determine if the issue is mechanical, electrical of air/fuel by watching the EKG.
Sorry my suggestions didn't pan out for ya.

[Stuka]

Put a timing light on it, make sure its not bouncing around. The vacuum bouncing during the miss is normal and can happen during any type of miss. Not just valve train related ones. I don't think you have a valve train issue based on what you say.

I am not familiar with that TBI conversion. Are you running a custom tune? Or has it ever been tuned?

Also, I moved this topic to Modified Tech, and may later move it to the EFI section if we determine thats the issue.

[TeaBag]

I have replaced the Dizzy, timing chain and gears, dizzy gear on CAM shaft, TFI Upgrade (plugs, wires, cap, rotor, Coil . Can't find it. I took the car in today to have them plug it into a scope. I'm out of ideas. I'll let you know what they say.

When I put a timing light on it and the mis occurs, the mark on the Harm Balancer jumps about 180 dgrs from where it should be.

Yes, I'm running a custom tune from AFI. besides the RPM jumping around and crappy fuel mileage (10.5 mpg), all WINALDL data looks normal.

[Stuka]

Well, I am inclined to think this is an EFI issue since everything else has checked out. Going to move this to Fuel Injection area. Maybe Bill will see it.

[TeaBag]

Here is the problem. Those were new plugs.

[Stuka]

You may lose some spark from that, does it run better with new ones?


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[TeaBag]

So far so good. Haven't driven it very far yet.