Rev 318 in park, slows down and stalls..case study.

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TylerW

TylerW

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Hey guys:

I thought I would post this as a "case study", since it's not technically on an A-body, but my '87 Plymouth Gran Fury. The backstory is that this is a very original car with 98K. When I bought it late last year, it had a terrible mid-range stumble that was predictable and consistent. Hold your foot in one spot and the car bucked and surged. It started easily and ran much better with the choke engaged. I rebuilt and installed a regular Holley 2280 carb which worked well in the past and built an HEI system to eliminate the ESC.

Fast forward to now: Starts easily, idles great, but has a part throttle flat spot severe enough to turn into a stall. I discovered that it's able to be duplicated in Park.

What I've done:
1. Removed the carb and replaced the linkage and baseplate with the factory items instead of the 6280 stuff and made sure the accelerator pump is working fully and the mechanical power piston is working fully and verified float height. No change.
2. Rechecked the timing. 10 degrees initial, mechanical and vacuum advances functional. No change.
3. Pulled the fuel pump inlet line and ran it off a bottle to eliminate the tank and lines. No change.
4. Pulled the fuel filter. Not clogged. No change.

That's where I'm at so far. Next up is put a clear filter in temporarily and observe fuel flow while prompting it to starve for fuel. There are always lots of could be's while diagnosing fuel problems, so maybe this will help.

Here's the video.

 
First 'hunch';
Fuel tank ventilation clogged up and pulling vacuum?
Wrong fuel cap on the tank (Vented/Non-vented)?

In these years the fuel tank ventilation probably goes through a charcoal cannister somewhere under the hood I would presume, which might be clogged up?
 
You said it sat around a lot , just asking did you put fresh fuel in it. Bad gas is always a problem .
 
You need to do a fuel deliver test.
To eliminate a supply side issue

The pump is working cuz it starts up again . But there is probably a supply side restriction or the pump is sucking air, or as mentioned; the tank is not venting, altho that is not on my radar, cuz it starts right up again with minimal cranking. My guess is the fuel level got too low for the low-speed jets. I don't think the bowl was empty cuz the crank-time was relatively short.
I had a similar problem when the rubber jumper from the sender nipple to the main line started sucking air; but the crank time was longer, and sometimes I had to wait a really long time.
But in your case I just think the pump is weak or there is a supply-side restriction and the the fuel delivery test will sort it out right quick.
 
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Does this have EGR? EGR is only active (open) during part throttle. closed during idle and WOT. are you bogging at the EGR tip in? You may have a pooched EGR diaphragm. Just pinch off the EGR vacuum line and test.

<<< just listened to the video and I bet (rebuilt carb?) your needle seat is flooding it. Thats why its a little cranky to hot restart. I had a friend who had a misadjusted regulator (wide open) and a black pump and it just overloaded the carb to the point where it would just gush out the booster and flooed it after about 30 seconds.
 
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Does the car still have a catalytic converter ?
 
Yep, sounds like a plugging up filter or a bad fuel pump.
Well it could be a couple of other things too, but that sure sounds like a fuel delivery issue.

It sounds like it runs just long enough to empty the carb at RPM's.
I think right after it dies I would first check the fuel shot, or even pull the carb and open it up.
I'd bet a dollar it's empty of fuel.
 
Here is an update after spending most of a day going through process of elimination stuff.

1. I did pinch the fuel return at the filter and there was no change.

2. I removed the gas cap. No change.

3. I removed the fuel pump inlet line at the frame rail and used my electric pump to pull fuel from the tank. It produced an ample volume of fresh, clean fuel.

4. I then plumbed in a cheap, clear plastic filter between the pump and the carb so I could observe whether the line is emptying during this sudden stall event. It is not. The filter remained full of gas at all times.

5. Finally, I revved the engine fairly hard, made it quit again and then pulled the top off the carb. Float bowl was full to the top.

To add a couple details....when you drive, the engine tries to die suddenly, but if you back out of it quickly and feather the gas it will recover and you can "drive out of it". It will not nose over afterward until you return to closed throttle and accelerate again.

The Electronic spark control is totally out of the picture. '79 vacuum distributor feeding an HEI module mounted on a heat sink, powered through a relay and controlling a TFI coil. The EGR is offline.

The main reason I've went through this diagnostic effort and didn't just start with the carb was to rule out assumptions and "must be's" and find the PROBLEM without throwing parts. It's a tough one. Hope you find this interesting.
 
can you observe the carb inlet when you are idling it to death? Youll be looking for the normal fuel mist to stop during choking or the boosters belching fuel and the motor flooding and quitting on you. you can blip the throttle when its flooding to clear it and drive through it as your flushing the carbs transfer slots that are "under water" when its flooding into a closed butterfly valve at idle. I would suspect a plugged muffler as a refusal to run at higher RPMs. I had a truck with a very weak electric fuel pump and I could idle it all day but once I got on it it would die in a matter of about 20 seconds. But it would always start right up. same would go for a blocked fuel filter. The guy at the auto parts store said the same thing, plugged cat or muffler so I cut them off to see if it helped, it didnt but I did find that my muffler had a huge leak and the pipe had pinholes everywhere so as to fail me in the smog test.
 
It was mentioned egr,and
There was no response. I would block it. Had one way back that was a nightmare.

If carb is full of fuel, then it could be spark. Bad pickup,worn dist shaft whacking pickup,air gap way off. Rotor shorting or a bad spark control.
 
Hey guys:

I'm sorry it's been a while, but here is the diagnosis video. Lots of good guesses, but they were just that..guesses...and everyone got it wrong. I assumed the wrong thing as well. Car runs excellent now once the issue was identified and corrected. Enjoy!
 
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