Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Mopar Big Block Talk
User avatar
RockyUSA
Posts: 1040
Joined: Sun Jul 03, 2016 10:10 am
Location: Tucson, Arizona, USA

Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by RockyUSA »

I am running a pretty stock 440 in my ‘72 Interceptor Mk 111

I have a Weiand Intake manifold, Demon 1900 carb, and Mopar electronic ignition.

When I am cruising I see very rapid indications of very lean conditions on my A/F gauge. Each one lasts for less than a second. These occur at cruising speed, about 65 mph. If I give the car more gas, they seem to go away.

I am convinced these are real, as in a couple situations on a long hill climb, these spikes coincided with a loss of engine power. On this occurrence they lasted for a couple seconds. The car was pretty hot at this time.

It seems to me that this must be carburetor, or a big, intermittent vacuum leak.

I have installed a new PCV Valve and Grommet, and also installed a Vacuum Gauge in the cabin, so I can try to correlate the spikes with a drop in vacuum.

Any other suggestions? I wish I had another carburetor to try out.

Thanks -

Rocky
Last edited by RockyUSA on Tue Apr 07, 2020 2:35 am, edited 1 time in total.
'72 Interceptor III - 133/5597
'73 DeTomaso Pantera
'74 Toyota Land Cruiser FJ-40
'95 Jaguar XJS Convertible
User avatar
jvcarrier
Posts: 525
Joined: Sat Feb 07, 2009 1:20 pm
Location: The Shires

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by jvcarrier »

Fuel starvation sucking small amounts of air into fuel line?
User avatar
RockyUSA
Posts: 1040
Joined: Sun Jul 03, 2016 10:10 am
Location: Tucson, Arizona, USA

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by RockyUSA »

That was also on my list of speculations, but I had forgot about that. Maybe the floats are set too low, and it's running out of gas (intermittently) at cruise?

But if that were the case, why would opening up the throttle more make it go away? It would seem like if it was on the ragged edge of fuel in the carb, it would bog if I gave it more throttle.

Anyway, another thing to check.

Thanks -

Rocky
'72 Interceptor III - 133/5597
'73 DeTomaso Pantera
'74 Toyota Land Cruiser FJ-40
'95 Jaguar XJS Convertible
User avatar
slotcarone
Posts: 939
Joined: Sun Aug 29, 2010 1:38 pm
Contact:

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by slotcarone »

Hi Rocky
Sounds like you are perceiving a problem that does not exist. You can't have an intermittent big vacuum leak. As for losing power when hot that could be the coil overheating. What drivability issues are you having? :)
1972 Interceptor III
133-5612
User avatar
RockyUSA
Posts: 1040
Joined: Sun Jul 03, 2016 10:10 am
Location: Tucson, Arizona, USA

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by RockyUSA »

You may be right.

The only real symptoms were the loss of power when climbing the hill on two occasions. This was tied to indications of lean operation lasting for a longer duration.

If the problem was ignition, the A/F meter would go totally rich, wouldn't it?

Rocky
'72 Interceptor III - 133/5597
'73 DeTomaso Pantera
'74 Toyota Land Cruiser FJ-40
'95 Jaguar XJS Convertible
User avatar
RayR
Posts: 233
Joined: Mon Mar 05, 2018 4:43 am
Location: New Zealand

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by RayR »

Hi Rocky,

Couple of questions;
- How lean is it going? 16/17/18 or off the scale?
- Does it do it at all cruise speeds? or only lighter throttle cruise?
- What has changed before it happened?

I had similar issues with my TQ Carb but not as sudden as yours sounds. On very light throttle cruise any small change of load would lean the carb for a second or so but any harder load and it was OK. I have put it down to transitioning between fuel circuits and have fixed with a variety of means, my TQ is very sensitive to orifice sizes in that are. It also had the same issue up a small hill but a more significant hill was no problem. Don't know how a Demon works though. PCV does also sound possible but you say you put a new one in after this happened?

Ray
MkIII Interceptor (140 8488)
User avatar
RockyUSA
Posts: 1040
Joined: Sun Jul 03, 2016 10:10 am
Location: Tucson, Arizona, USA

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by RockyUSA »

Couple of questions;
- How lean is it going? 16/17/18 or off the scale?
- Does it do it at all cruise speeds? or only lighter throttle cruise?
- What has changed before it happened?

Hi Ray -

Thanks for thinking about this.

I just have a cheap A/F Gauge (Sunpro 8200), with no actual readout. It's just red (lean), yellow (ideal), green (rich). So I can't give you a real number, but it will spike to off the scale in the red.

It seems to do it at a single speed - but it's at a lower throttle setting. It seems like if I give it enough gas to make it stop doing it, I am in an acceleration state.

I'll get some more data next time I get to the freeway - I have a throttle position sensor on my transmission controller. That might give me more concrete information on when it's occurring.

I can't say that anything significant had changed - this behavior may have been going on all the timer. I have been working the ignition (FBO Timing Advance Plate, Set timing to 16* static, 36* total) and had been monkeying around with jets and metering rods to try and get the motor to run more in the "Ideal" range, as of now it runs in the bottom end of the "Rich" zone. With the timing and carb changes I have about 12"-14" of vacuum at idle.

I just installed this gauge in the cabin - maybe it's just making me paranoid!

Thanks -

Rocky
'72 Interceptor III - 133/5597
'73 DeTomaso Pantera
'74 Toyota Land Cruiser FJ-40
'95 Jaguar XJS Convertible
User avatar
slotcarone
Posts: 939
Joined: Sun Aug 29, 2010 1:38 pm
Contact:

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by slotcarone »

Hey Rocky your last sentence says it all!! Stop watching the unnecessary gauges and just enjoy the ride!! The sound of the exhaust should be drowning out these thoughts!! :)
1972 Interceptor III
133-5612
kenny38
Posts: 647
Joined: Tue Aug 25, 2015 8:24 am
Location: Australia

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by kenny38 »

I have had a similar problem for years. On a hot day (especially driving in to sun) car going well than after say an hour or two .....fuel starvation hic cupping slowing me down to 60k and some times cutting out causing me to pull over to side of road. Car would immediatly start and would go OK for another 10 mins or so and then same again. I have fitted super new electric pump as advised. I have tracked the fuel line all down the car and it is not touching exhaust. My mechanic had a look and said it didn't make sense as there was nuffin he could see. However he has suggested a look under the carb and perhaps put something like asbesto covering there. He has to source the asbesto. Hope this will work. I haven't driven her since bringing her back from Grafton on a truck after the strange affair of the deisel fill up. The tank was drained for the first time since car was new so maybe if trouble was in there it's now fixed. 8) It's all a friggin nuisance. Kenny38
User avatar
slotcarone
Posts: 939
Joined: Sun Aug 29, 2010 1:38 pm
Contact:

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by slotcarone »

Hi Kenny
Sorry to hear about the diesel fiasco!! Concerning the engine problem I feel you are wasting your time with the carb insulator deal--from what you are describing that is not the problem as it would not start right up again if that was the cause. Are you sure it is not an ignition issue? That is where I would be looking. Sounds more like a coil issue. Is your car modified and what do you have for the ignition system. I also had a similar problem which drove me crazy and it turned out to be bad connections at the ignition switch. It seems the switch is held into the housing by a small pin that allows it to move very little when you turn the key. My car would cut out and stop and then on the side of the road it would start right up again and run for a short time then die again. The small amount of movement was re establishing the connection and allowing the car to run again. You could possibly have a similar issue. :)
1972 Interceptor III
133-5612
Joe Schiavone
Posts: 699
Joined: Wed Apr 30, 2014 7:28 pm
Location: United States

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by Joe Schiavone »

Chuck. With the engine basically stock and not all new parts I would pull the plugs and read them. Adjust the heat range of the plugs for the ones reading a bit dark and take the car for a nice top end run and enjoy. Then pull the plugs again. Do not focus too much on a split second situation unless you have all new elec and mechanical parts. Too many variables Rather
User avatar
RockyUSA
Posts: 1040
Joined: Sun Jul 03, 2016 10:10 am
Location: Tucson, Arizona, USA

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by RockyUSA »

Thanks, Joe.

Again, sage advice. I do need to check my plugs since I made all my changes to my carb jetting.

Thanks for the suggestion.

Chuck
'72 Interceptor III - 133/5597
'73 DeTomaso Pantera
'74 Toyota Land Cruiser FJ-40
'95 Jaguar XJS Convertible
User avatar
RockyUSA
Posts: 1040
Joined: Sun Jul 03, 2016 10:10 am
Location: Tucson, Arizona, USA

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by RockyUSA »

Just for you guys (and gals) who have been following this saga with baited breath - the issue is real, and it got worse yesterday.

Intermittent lean spikes were occurring to a more significant magnitude through out a day of driving. I took some video - I'll try and post it.

On my way home, about 1-2 miles from the house, the car would go totally lean, and loose power. By pumping the throttle, I could get it to intermittently "pick-up" and run for a few tens of yards, before it would loose power again.

The car stopped about a mile from my house (luckily). I got a quick tow home.

Immediately after getting home, I demonstrated that this loss of power was due to fuel starvation, by immediately starting the car with a little gas poured into the carb.

When I pulled the fuel line off the carb, it was dry.

About 2 hours later, I got the car running, and it's back running normally again. This is the same symptoms that I had a couple months ago.

So what I have done now is:

1. Replaced the fuel filter with one without a fuel return.
2. Blocked the fuel return hose.

I took it for a drive - still seeing the lean spikes. So I am beginning to be convinced that it is likely a fuel delivery problem - probably caused by low fuel pressure from my stock fuel pump.

I am going to get a fuel pressure gauge on my carb first, and then try a more super-duper fuel pump.

Stay tuned.

Rocky
'72 Interceptor III - 133/5597
'73 DeTomaso Pantera
'74 Toyota Land Cruiser FJ-40
'95 Jaguar XJS Convertible
User avatar
Steve Payne
MASSIVE RED CARD
MASSIVE RED CARD
Posts: 5701
Joined: Thu Nov 17, 2005 4:51 pm
Location: Witney , Oxfordshire
Contact:

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by Steve Payne »

This fault could be a couple of other things before you go changing the pump.

Is air getting into the fuel tank to replace the fuel coming out? Easy to check, run the car for a while and when it starts spiking lean remove the filler cap and see if the problem persists.

I have seen fuel pipes partially collapse especially as they get hot after a run. On one car the fuel filter was crumpling while the engine was running due to the suction.

The other problem I have seen is the pushrod for the fuel pump has worn over the years, when measured compared with a new one I found it was several mm short so the fuel pump was not getting fully depressed.

Steve
JOC official Lightning engineer
User avatar
RockyUSA
Posts: 1040
Joined: Sun Jul 03, 2016 10:10 am
Location: Tucson, Arizona, USA

Re: Intermittent “Lean Spikes” on Air/Fuel Ratio Gauge...

Post by RockyUSA »

Thanks, Steve -

I have a fuel pressure gauge and fittings on the way for "a basic" look at the system.

I don't know that my wife will enjoy the ride on top of the motor watching the gauge as I accelerate!

Anyway - I need to look for this pushrod in my documentation - I am more used to the Ford-style mechanical pumps that just have an arm that rides on an eccentric. Here are some specs on said pushrod..

Possible Options: Comp Cams 4626 (Chromoly Tip) / 440Source 125-1020 (Comp Cams)
Length: 3.220"
Diameter: .432"

(I'll search here after I finish this post and see what I can find)

I was "watching" the Carter M6903 fuel pump but didn't want to pull the trigger until I make sure I've got enough pressure (or don't).

Again - thanks for the tips. I can easily check the gas cap thing... Good suggestion.

Rocky
'72 Interceptor III - 133/5597
'73 DeTomaso Pantera
'74 Toyota Land Cruiser FJ-40
'95 Jaguar XJS Convertible
Post Reply

Return to “V-8 Engine Tech”