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Smart quod bastardus
 
fredmeister's Avatar
 
Join Date: Oct 2002
Location: Baltimore, MD, USA
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Quote:
Originally Posted by 930cabman View Post
Maybe "misfire" is the incorrect term. The issue occurs when under boost, about .4 atm. I will be rolling along just fine, increase the throttle opening to a point where the turbo kicks in, she pushes ok until it reaches about .4 or .5 atm boost. If I keep my foot into it a rather strong hesitation occurs usually accompanied with a backfire in the intake system.
Given these results I am no longer thinking about the plug wires/plugs.
Is the issue somewhere in the wastgate?
I am currently at a loss. She runs just great until I decide to give her a kick.
Could it be your overboost switch is kicking in too early and causing the engine to shutdown at 0.4bar instead of 1.1-1.3bar?
Is this a failure anyone else has ever seen? I have not seen too many posts here on this but its a possibility.

I am not sure the exact spec on the pressure switch when its supposed to ground the fuel pumps==== anyone know is it 1.1-1.3bar or something else?

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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
---"When you're racing it's life! Anything else either before or after, is just waiting"
Old 05-21-2012, 09:00 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #21 (permalink)
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YES! More info...

Very good description of your problem. Very good points brought up by many. Forget the ignition at this point, provided the timing is correct, (likely so)

Fred has a real good point bringing up the overboost function, although usually when it kicks it, the interuption in power to is sudden for a lean condition long enough to cause a backfire. Also, I have never seen one change pressure settings from wear. 1.3 or so is the right where they kick in.

Checking CIS pressure is a great idea, provided you have the gauge. However, that is only half the story in testing your fuel system. The gentleman that mentioned one of his pumps failing was experiencing a fuel Volume problem.

The CIS system has an enormous amount of fuel volume flow compared to an EFI system due to the fact that is uses fuel pressure to control mixture via the fuel distributor.

It is common to test the pressures in the garage and all seem well and good, because you are not out under a load creating the same conditions at which you experience trouble.

The best way to test fuel volume is per the factory manual. It states to measure the fuel flow AFTER the entire system while it under pressure. This is done by disconnecting the fuel return line at the hard line on the way back to the tank. You probably need to connect o piece of hose to the metal pipe coming from the engine somehow to route it to a suitable container for measurement.

If you don't have the volume, you won't have the pressure, so it's a half@ss test to only check volume, but if this is likely you problem and you simply want to confirm it without procurring a CIS test gauge, there is no harm in checking volume alone. Again, if the vloume/flow tests pass, don't assume the pressures are good, but if they fail, start looking into fuel supply before bothering with a gauge. The most common easy problem is a bad fuel pump relay, next there could be an issue at the fuse block, there could be a bad pump, or there could be a restriction in the supply to the front pump from the fuel tank. There is a screen in the bottom of the tank, as well as some passages that can get stopped up.

If both pumps are running, one under middle of front axle centerline, one under car in front of LR wheel, but making ALOT of noise, check the supply to the front pump from the tank. They are noisy from cavitation from trying to pull fuel from the tank through the screen, passages.

If only one pump is running and making ALOT of noise, figure out which pump isn't running and why. Power? Ground? Dead pump?

I hope this is your problem as it is much easier to solve a volume issue typically than diagnosing a pressure problem, IMO

Good Luck,

Mike
Old 05-21-2012, 10:36 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #22 (permalink)
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Mike beat me to it...

Reread Blaine's #13 post. Our 930's require lottsa fuel pressure and volume. Check those first. Then, unplug the green conn on the airflow sensor and with key on/engine off, get the fuel pumps running. Wiggle the wires on the fuel pump circuit, tap on the fuel pump relays, and wiggle appropriate fuses in their holders. Do the pumps still run without going intermittent? Feel for excessively hot pump relays and warm wires. Is rear pump getting full battery voltage? Clean conns, if necessary.

Then, check CIS pressures. My $'s on a bad rear fuel pump.
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Old 05-21-2012, 10:48 AM
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My bet is on a faulty rear fuel pump as well

I went through all the above steps (o-rings, plugs, fuel filters, over boost relay, etc.....) none of these items fixed the problem. If I didn't accelerate too quickly, I could get the car to rev up to 6K rpm (low fuel demand), but when I put my foot into it, she cough hard at 4k rpm's and only recover if I lifted.

$350 for a new pump and I've never had a return of the issue.

If you get it resolved you must chime in with what fixed the issue.....it helps everyone out.

Good luck,

Blaine
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Old 05-21-2012, 04:30 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #24 (permalink)
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What a great forum, all of the responses, well I am impressed. The only issue I can forcast is TIME. I have owned a fuel pressure gauge for 25+ years, but never used. I kinda suspect she may be running out of fuel, which could point to a faulty pump. With a little luck I will be able to get back into her by the weeks end.

Thanks again, all of you loyal 930 guys (and gals)
Old 05-21-2012, 05:12 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #25 (permalink)
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By your second more descriptive symptoms, I would checked the WUR. May be borrow one for a test - only - with another near by Pelicanee? It only takes few minutes to swap a WUR.
Old 05-22-2012, 07:53 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #26 (permalink)
 
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fuel pump(s)

Is testing the fuel pump output as simple as collecting the gas in a container for a specified amount of time and measure the fuel collected? Also, why do we have two fuel pumps? does the rear pump pressurize for the CIS system and the front pump supplies for the rear pump?
Old 05-23-2012, 08:07 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #27 (permalink)
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Pretty much. The hardest part is getting the return line loose and fitting a rubber hose over the flare fitting on the metal line from the fuel dist.

You wanna catch the fuel from the line with a "Y" at the end. One hose goes to the bottom of the fuel accumulator to catch and return any fuel should the diaphram in it ever rupture.

The other line goes to the front of the car, It points more forward. This is where you want to collect your sample, from the line coming from the fuel head obviously...

I believe the minimum spec is something around 1100cc in 30 seconds?? It's been a while hopefully someone knows it off the top of their head... I know a healthy system will typically double that number...

you test with the engine off, key on, I usually leave the airflow plate switch plugged in and ground the overvboost switch to turn the pumps on and off, it's a litttle safer.

Ideally, you want to know the pressure is in spec to verify volume, testing this way ONLY verifies a low fuel delivery problem, but it is very simple and doesn't require a CIS gauge to eliminate one likely cause of your symptoms.

If all is well, start looking elsewhere (pressure test, WUR function, etc)

In my experience a bad WUR usually caused other noticable drivability concerns, especially when cold... You haven't mentioned any other symptoms so I'm stuck on the flow test...unless you have other symptoms and unknowingly chalk it up to the "nature of the beast" as many others do...

I assume you've verified both pumps are running, do this first if not.
Good luck
mike
Old 05-23-2012, 09:50 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #28 (permalink)
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Yes, both pumps are running. Can the location for measuring the fuel volume be taken from the output of the rear pump? or a Y (?) connection at the fuel distributor. Where is the overboost switch?

thanks
Old 05-23-2012, 04:01 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #29 (permalink)
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fuel volume

I was able to collect the fuel at the return line from the "Y" connection earlier today.. I ran the pumps for 30 seconds and could only manage about 600cc. I did not have a pressure gauge hooked up, only collected fuel for volume. I was not impressed with the output, it seemed weak. I may be able to borrow a CIS pressure gauge and test that.

Old 05-29-2012, 04:44 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #30 (permalink)
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