possible cause of miss/lean

T77911S · 10-29-2013, 05:42 AM

i have had a very slight miss at cruising speeds and it has also been lean at cruise.

while waiting on parts to finally be returned after 3 months, i was finishing up up the car last night.
to keep this short:
back in the left corner of the engine bay there is a bracket on the wall that the 2 fuel lines run through along with the vac line to the booster. i was looking at it when i noticed the return line appeared to be pinched. so i got the lines moved around so it was not pinched any more. but, could this cause the fuel pressures to vary? perhaps raising control pressure when there is more fuel to return to the tank making it run lean at cruise and richer under boost when less fuel is returned to the tank? just being hopeful.

the other thing i found is a bad ground on the bracket for the coil. yes, i checked this several times...but, it always had wires on it and yes, i did think i had a bad ground and redid the connections, but! the stud on the fan housing is bad!
i was going to run my fuel pumps last night to check my wiring and fuel flow. when i tried to ground the over boost wire onthe stud towards the front of the car, no run. ground it on the other stud and they ran!. and yes, the PO bypassed the over boost and had the wire grounded on that post. the only difference now is that bracket is now powder coated and perhaps i was getting some ground signal through the front stud and braket to the back stud.

this has been the best looking most consistant option for fixing this.

you know, this could also be part of why it would only run when i grounded the case on the CD unit, but then next day it would run without grounding the case.

i am thinking i may ground the IC.

should have parts wed!!!

mooney265 · mooney265's Garage

With the cold [more dense] air just coming in, if your car was setup prior to run "almost lean" , then it will all of a sudden begin to run more lean with the change in climate...

I used to have mine setup right on the edge [14.8-14.9], and whenever a cold front would move in, it would get a slight miss at cruise [15.2+]. Now, I've got a couple points and run 14.5 on cruise and when it gets cold, I go up to 14.8...

AFR's I'm referring to...

T77911S · 10-29-2013, 09:12 AM

i was originally in the 16's at cruise. 13 at idle and 12 on boost.
not had it long enough and especially with it off the road for 3 months waiting on my WUR i have not driven it much.
i am also fixing air leaks into the engine bay

wjfk32 · 10-29-2013, 12:23 PM

Quote:

Originally Posted by mooney265

With the cold [more dense] air just coming in, if your car was setup prior to run "almost lean" , then it will all of a sudden begin to run more lean with the change in climate...

I used to have mine setup right on the edge [14.8-14.9], and whenever a cold front would move in, it would get a slight miss at cruise [15.2+]. Now, I've got a couple points and run 14.5 on cruise and when it gets cold, I go up to 14.8...

AFR's I'm referring to...

Wouldn't your afr readings be more "Rich" as the weather gets colder??? My readings are a bit richer at cruz (10-15 degrees colder) and I did have a slight hesitation now and then (no worries). Weather in NYC area weekend were in mid 50's or so.. Top end power was endless.

Walt

mark houghton · 10-29-2013, 07:06 PM

Quote:

Originally Posted by wjfk32

Wouldn't your afr readings be more "Rich" as the weather gets colder??? My readings are a bit richer at cruz (10-15 degrees colder) and I did have a slight hesitation now and then (no worries). Weather in NYC area weekend were in mid 50's or so.. Top end power was endless.

Walt

Walt Walt Walt....did you leave your thinking cap at work today?

Cold air=more dense air=more air mass mixed with the fuel=leaner mixture/higher AFR readings.

But the question now is this: Will that extra mass of air - being so dense and actually weighing more than warmer air - will it cause the fuel metering plate/arm to deflect further (that mass exerting more force upon the plate...after all, my early courses in physics said that force=mass x acceleration, so if the speed of the air remains constant but the mass increases, then the force exerted on the metering plate increases). If that happens, then the fuel distributor will deliver more fuel to compensate, and the AFR's won't change. Hmmmmmm........leave it to me to twist the $hit outa things.

jsveb · jsveb's Garage

Quote:

Originally Posted by mark houghton

Walt Walt Walt....did you leave your thinking cap at work today?

Cold air=more dense air=more air mass mixed with the fuel=leaner mixture/higher AFR readings.

But the question now is this: Will that extra mass of air - being so dense and actually weighing more than warmer air - will it cause the fuel metering plate/arm to deflect further (that mass exerting more force upon the plate...after all, my early courses in physics said that force=mass x acceleration, so if the speed of the air remains constant but the mass increases, then the force exerted on the metering plate increases). If that happens, then the fuel distributor will deliver more fuel to compensate, and the AFR's won't change. Hmmmmmm........leave it to me to twist the $hit outa things.

Yes

Drag= 1/2 rho *V^2*S*Cd

However, remember speed is not the same as acceleration.

Interesting perspective Mark

mark houghton · 10-30-2013, 06:05 AM

Quote:

Originally Posted by jsveb

Yes

Drag= 1/2 rho *V^2*S*Cd

However, remember speed is not the same as acceleration.

Interesting perspective Mark

Absolutely correct, I was wondering if someone would catch that. Unfortunately it's been a zillion years since college physics and I've forgotten more than I ever learned.

mooney265 · mooney265's Garage

Whatever the reason, my AFR gauge [Bosch Wideband] reads leaner on colder days than warmer days - - all other things being equal...

wjfk32 · 08:59 AM

U r correct

my bad.

So as the air temperature decreases, to keep the proper air/fuel ratio you would be required to richen the mixture (add fuel).

If the air temperature increases, to keep the proper air/fuel ratio you would be required to lean the mixture (decrease fuel).

My AFR-Readings at Top End. 6000rpms are reading a little a slightly bit better (richer)..Go figure..

Maybe its that Plate Theory Thingy...

Then I am running a 7th injector.. Haven't changed the controls on it.....

T77911S · 10-30-2013, 09:30 AM

post #2 sure took this off into left field. where did i ever say anything about air temp?
i was talking about a pinched fuel return line and a bad ground

mark houghton · 10-30-2013, 11:58 AM

Quote:

Originally Posted by T77911S

post #2 sure took this off into left field. where did i ever say anything about air temp?
i was talking about a pinched fuel return line and a bad ground

Yes....well....that happens sometimes doesn't it? Perhaps Mooney was thinking that with the colder autumn temperatures upon us - and since it's been 3 months since you last ran your car - you might very well be a bit leaner just due to the temperature change in the 3 months since.