Fuel pressure question, requires expert
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Joined: Jul 2005
Posts: 9
Likes: 0
Fuel pressure question, requires expert
85 L-98 adjustible fuel pressure regulator.
If I set the pressure any more than 36 psi I get some black smoke and soot on the back of the car and the ecm gives a lean exhaust code. If its set under 36psi there is no lean code. To add to this mess, I have a Nordsog digital air/fuel ratio guage, that under both cases reads lean except when at idle. The 85 shop manual states 39psi stock pressure, and many claim to run up to 47psi. Whats the deal with this.
i'm sick of cleaning the back of my ride, THANKS
If I set the pressure any more than 36 psi I get some black smoke and soot on the back of the car and the ecm gives a lean exhaust code. If its set under 36psi there is no lean code. To add to this mess, I have a Nordsog digital air/fuel ratio guage, that under both cases reads lean except when at idle. The 85 shop manual states 39psi stock pressure, and many claim to run up to 47psi. Whats the deal with this.
i'm sick of cleaning the back of my ride, THANKS
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Do you have any other mods to the engine?
Are you reading pressure at idle with the vacuum hose disconnected?
Is the ECM throwing any codes?
Jumper ALDL pins A&B (upper right) and start the engine. After a minute or so the SES flash rate should slow from 2Hz to 1Hz when the ECM goes into closed loop mode. Further the On/Off flash durations indicate righ/lean condition.
If it checks out OK at idle try several steady RPMs to see if the flashes settle to equal on/off durations at each steady RPM.
Are you reading pressure at idle with the vacuum hose disconnected?
Is the ECM throwing any codes?
Jumper ALDL pins A&B (upper right) and start the engine. After a minute or so the SES flash rate should slow from 2Hz to 1Hz when the ECM goes into closed loop mode. Further the On/Off flash durations indicate righ/lean condition.
If it checks out OK at idle try several steady RPMs to see if the flashes settle to equal on/off durations at each steady RPM.
Advanced
Joined: May 2005
Posts: 66
Likes: 2
This is probably no help but I swapped out my fuel pump last night and the car is a bone stock '85...
The pressure was a very steady 32 at idle and a very steady 28 at 3000 RPMs and that's with a brand new gauge. I got the exact same thing with the old pump and the new pump - but I needed to swap out the pump anyway because it was getting noisy and the little sock on the end of it was pretty beat and seemed very restrictive - I could barely blow through it... Filter is brand new too...
So maybe someone can explain why this car has such low pressure? - it seems to run fine this way - will still run around and light up the tires and all - I do seems to have a slight miss in the midrange but the ignition stuff is old and I'm replacing all that next just to get the car up to par after 20 years...
I'm wondering if the '85's have a different size injector and like to have less fuel pressure? If so - maybe that is why when you crank up your pressure your getting the black smoke like it's super rich?
The pressure was a very steady 32 at idle and a very steady 28 at 3000 RPMs and that's with a brand new gauge. I got the exact same thing with the old pump and the new pump - but I needed to swap out the pump anyway because it was getting noisy and the little sock on the end of it was pretty beat and seemed very restrictive - I could barely blow through it... Filter is brand new too...
So maybe someone can explain why this car has such low pressure? - it seems to run fine this way - will still run around and light up the tires and all - I do seems to have a slight miss in the midrange but the ignition stuff is old and I'm replacing all that next just to get the car up to par after 20 years...
I'm wondering if the '85's have a different size injector and like to have less fuel pressure? If so - maybe that is why when you crank up your pressure your getting the black smoke like it's super rich?
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Melting Slicks
Joined: Jun 1999
Posts: 2,949
Likes: 23
From: Bowling Green KY
Originally Posted by Redeasysport
Sounds like a bad O2 sensor to me. Check your codes.
Le Mans Master
Joined: Apr 2000
Posts: 6,341
Likes: 1
From: Shelton CT
Originally Posted by winston121
I must have overlooked that. thats the best answer so far, only the 85' came with 26lb injectors, all the other L-98's I believe were 24 and lt-1 were 22 stock. thanks
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
If your gettin Lean codes and black smoke(rich) is coming out your exhaust then the O2 needs checked.It has nothing to do with the injectors under 40psi the computer should learn but it is not doing it. Get a Datamaster scan done.
Melting Slicks
Joined: Apr 2001
Posts: 2,120
Likes: 222
From: Ca.
I always suggest everyone buy a scan tool-even the autoxray will work well, and is fairly cheap and is about a must have for real diag.
Scan it and check for codes-then scan live data-look for o2 sensor voltage to swing back and forth rapidly; also the o2 counts should be rapidly increasing as it runs. Also make sure that it is going closed loop. If none of the above is happening-throw in a new o2 sensor first.
Look at blms-should be 128+/- 4 or so.....
Scan it and check for codes-then scan live data-look for o2 sensor voltage to swing back and forth rapidly; also the o2 counts should be rapidly increasing as it runs. Also make sure that it is going closed loop. If none of the above is happening-throw in a new o2 sensor first.
Look at blms-should be 128+/- 4 or so.....
Intermediate
Joined: Jan 2004
Posts: 45
Likes: 0
From: Elmsford NY
My '85 likes higher fuel pressure. I think I set it in the high 40's. I don't remember exactly. As other people have mentioned, checking the O2 sensor sounds like a good place to start. Soot out the exhaust usually indicates a rich condition, yet the computer is setting a lean trouble code.