Bruno DeSousa

1981 928 US Ljet 5 spd... im trying to track down my running rich fuel issue. just a quick question about the fuel damper on these cars... i have changed the vacuum hoses everywhere.. when i test for vacuum my gauge reads about 20psi at idle 700ish RPM... according to the service manual i should be seeing 30ish PSI... correct? i dont know why the pressure seems so low.. anyways when i pull the vacuum hose off the nipple on the fuel damper there seems to be no change in idle or performance.. when i hook up my vacuum tester to the damper its holds pretty well about 25PSI on the gauge.. is it possible that the damper is still malfunctioning even tho it seems that the diaphram is holding pressure? the two FPRs at the other end of the fuel rails hold pressure too.. hmmmm

Bruno DeSousa

maybe they are stuck shut? just thought of that.. the car was sitting for a long time and idk perhaps the damper got glued up with old gas? i know i should do a fuel pressure test .. im trying to track down the correct fitting for the fuel rail test port.. i should have it tomorrow...

thanks guys for reading

James Bailey

The front damper is a simple rubber diaphragm to smooth out pulses...the vacumn line does nothing but provide a safe fuel path if the diaphragm breaks....better to dump fuel into the intake rather than all over the top of the motor......so as long as no fuel is in the vacumn line all is good..

Bruno DeSousa

idk man.. i cant figure out why the car is running extremely rich!! i checked just about everything... uhhhhgggg! does 20psi manifold vacuum sound good at idle 700ish RPM? i dont know where else it could be leaking from, i replaced all the vacuum hoses / checked the spider boots and all other rubber lines etc... hmmmmm

would a bad set of plug wires be causing this issue?

thanks for reading

Bruno DeSousa

1981 Ljet 5spd US...

been running extremely rich!! i had to change the oil just cause it was sooooo thin from the gas blow by.. the oil was about 1 month old(changed by the PO before i bought the car).. i just been trying to get it running correctly in my driveway. i have not been driving the car on the road.. i literally drained 10+ quarts from the oil pan... this means there was about 2-3 quarts of gasonline blow by in the crankcase.. this is insane! im running extremely rich and i cant figure out why!!!??? i need to do a fuel pressure test asap.. but there is no gas in the vacuum lines soo i dont think the FPRs are leaky BUT IF they were leaking wouldnt that cause the fuel pressure to be lower at idle rather than a stuck closed FPR that would do the opposite meaning raise the fuel pressure and cause a running rich issue... im just as confuses as you after reading that paragraph.. lol

thaks again for reading

Mrmerlin

remove the fuel injectors and send them off to witch hunter for cleaning

WallyP

Vacuum is usually measure in inches of mercury, not psi. 20" at idle is pretty high (which is good), and will usually have nothing to do with running rich.

The only time that intake vacuum will reach 30" is on over-run - that is, when the throttle is completely closed as you are coasting.

Are you confusing fuel pressure and manifold vacuum?

GlenL

Or is it coolant in the oil?

Bruno DeSousa

the title is "bad plug wires cause running rich / poor idle?"

injectors been cleaned already.. def gas in the oil.. coolant would make it look like milky.. which it isnt.

Bruno DeSousa

no im not confused.... i havent taken fuel pressure yet..

hacker-pschorr

The ignition system has no effect on the amount of fuel entering the cylinder, just how much of the added fuel is ignited.

From what you've posted:
1. Too much fuel is being sprayed into the combustion chamber
---or---
2. Your ignition system is faulty in some way causing it not to burn all the fuel.

If #1, culprits in no particular order are:
1. AFM / Barn door
2. L-jet Brain
3. Coolant temp sensor
4. FPR / Fuel Pressure

Easiest one to check is #3, just need a multi-meter and follow the specs in the WSM which I can post up later if nobody else does (don't have that file in front of me).
The other two, you really need a "known good" donor to test before and after.

If #2:
1. Green Wire (goes from passenger side of engine compartment to distributor)
2. Ignition brain (inside fender, passenger side)
3. Any other component in the ignition...(plugs, wires, cap, rotor, coil etc...)

Have you pulled the plugs? You mentioned you already had the injectors cleaned. Years ago so did I and the car ran stupid rich. Pulling the plugs I discovered one was saturated in fuel, the rest were fine. Turns out one of the "cleaned" injectors was broken. I had this happen twice, the second time the bad injector didn't spray at all.
So don't assume that just because you have them cleaned all 8 injectors are fine. Pull the plugs, compare the pattern. If there are any variances those cylinders have unique issues that are tied to the injector or the harness. These are a batch system so all should fire at the exact same time and the same duration.

If the plugs all look the same, I would start with the Coolant Temp Sensor. It's the easiest part to check of all the above and stupid cheap to replace.
This sensor controls the injector pulse width, if the resistance is too high, the injectors will stay open too long at every point in the engine cycle (idle, acceleration, cruise etc...)



There is also the idle and WOT switch on the throttle. I doubt it's these since the idle switch would only cause isle issues...and the WOT switch not working causes the engine to run lean upstairs.



Hopefully I didn't forget anything, I'm a bit rusty on L-jet.

hwyengr

Are you confusing the vacuum spec, though? Because the number you've mentioned sounds a lot like the fuel pressure specification. Especially since vacuum usually isn't measured in PSI.

Assuming it's 20 in-Hg and not PSI, that's really good. 20psi of vacuum would be way too high, since that would be, like, 40 in-Hg.

Bruno DeSousa

ahhh i see.. i just checked my test gauge.. it is in (in-HG) .. soo i guess my vacuum is good.. BUT why in the service manual it refers to manifold vacuum in PSI? unless i mis-read it ..

anyways ok soo assuming my vacuum system is working now... what else would cause HIGH fuel pressure? (stuck closed FPR? clogged return fuel line?) hmm

thanks for reading and responding!

mark kibort

Pressure is determined by the FPR.... If that return line is blocked, that will increase pressure to the capability of the pump. the injector conditions have little to do with the result pressure. it should be fixed whether the car is running or not, and based on the fuel pump running.

mark kibort

you are checking system vacuum, not fuel pressure . huge difference. 30psi is fuel pressure and you need to have a gauge on the fuel line at the front, not on the vacuum lines... the vacuum has only one effect on the fuel pressure regulator, in that it reduces the fuel pressure by letting more fuel return to the tank during off throttle.... i dont even use that line and its fine and no ill effects but bad MPG (but its a race car so , so what).