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930 Turbo Hesitation/ Bucking

I recently started having a hesitation/bucking problem on full acceleration! The timing is perfect, the plugs are great, the distributor is perfect, the plug wires are new, the ignition coil is great, the fuel injectors were poptested and all passed, the WUR is new, fuel distributor is rebuilt! Upon starting the car it feels like the cold start valve helps to relieve the hesitation. I can rev up the car and there is no hesitation at all until the thermal time switch shuts off the cold start valve then that is where the hesitation is noticeable! The idle is rock solid when the engine is cold or hot! The rear fuel pump is a brand new unit! But the one in front is a rebuild that was put in 2.5 years ago! The rear fuel pump works perfect. Oddly the front fuel pump upon starting the car does not seem to activate the red relay in the fuse panel as it used to! I seem to only feel the hesitation/ bucking when the car is being driven around 2000rpm to 3000rpm and then it clears up! I believe that if this was an air leak issue then I'd see a hunting idle! And if the CO% was out of place then the idle would be osculating! But in my case the idle is rock solid!

Also if one of the cylinders was leaking air, could the idle still be rock solid, or would it affect the idle? Would it only affect the drivability on load?
Old 12-23-2006, 10:42 AM
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Have you tried using CIS gauges to check cold, warm & hot pressures ? (or maybe it is only hot & cold pressures, I'm not sure.)
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg
Old 12-24-2006, 12:49 AM
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Madkow,

I'm no 930 plumbing expert. BUT bucking under loads at certain rpms makes me concerned of AFR's. Rev's in neutral are more transcient and more forgiving without load. Cold start settings are purposely running at richer AFR's and therefore might cloak a lean condition. When under "full acceleration" your under most load and typically more boost. These are both going to be conditions most sensitive to AFR's.

You upgraded/restored a lot and though we often question parts failures, could it be more likely simply adjustments out of range?

I would think a vacuum leak would effect idle. Vacuum leak symptoms get better under full throttle. I agree that a pipe/hose leaking would likely be more broad in areas of effect.

I say, get hold of a broad band O2 or dyno. If your brave, adjust the CO2 to the richer side in this rpm range and see if hesitation improves?

If a cylinder was leaking this bad I think you'd see other symptoms. That plug might show issues. You could check compression. I think you could determine by selectively checking by eliminating clyinders. I would think that it might be worse when cold as tolerance usually work in your favor as she warms up.

How long has it been assembled? Has this issue came about out of blue or after changes? What is your skill level and how hands on are you?

Merry Christmas, All!
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Old 12-24-2006, 07:03 PM
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I have tried enriching the fuel, but no improvements. So here's the story! All this really started with a slight misfire which my mechanic mentioned was an electrical problem about a year ago at the same time cylinder 6 had this odd symptom of loosening up the sparkplug and disconnecting the wire connection itself. Just last month I had time to work on the car. Looking for sources to the issue I took it upon myself to replace all of the seals in the intake. When that didn't solve anything. I had the fuel distributor rebuilt. I then adjusted the valves on the car which was over due. Opening valve covers I discovered a barrel nut that had loosened itself and was just in there. I put that nut back into it's place and the car ran great! I took it out for a drive that day and came back with a rough running engine and a squeeking cylinder 6. I took apart the valve covers on the right side and re torqued all the barrel nuts, and the sound went away, but the car still ran like crap. So I took apart the valve covers and rechecked the clearances and they all checked out! I put a new ignition coil in, wires and spark plugs next. There was no improvement. I then aquired a throttle body for the car. The car came with a throttle body that was damaged. The idle screw was stripped and someone put teflon tape on it! I checked the numbers on the throttle body for a match and they matched perfectly. The car ran better but still had a misfire! So then the ignition distributor was next on my list! I was able to find a distributor on ebay. I took it apart and it looked a lot better than the original which I rebuilt. I put a new pulse coil in it and installed it. The car then idled perfectly. But now it has the hesitation like it doesn't have enough gas going to it. Then the next day the perfect idle somewhat went away. I really haven't run the car for a year. I checked all the injectors on a pop tester and they all checked out except for one that was leaking and I replace that one after looking at the spark plugs and realizing that one was wet with fuel. The car then idled great =) But it was decieving! I took the car out for a drive and then, that is where the bucking became evident. I noticed a sound inside the cabin as if the car was straining to get fuel! It sounded like the front fuel pump seizing! So the next day I bought a new fuel pump from Performance Prod. It was one of those pierburg pumps. I put it in and just when I opened the clamp to let the gas go through a drip started to develop! I checked to make sure it was non of the fittings! It became evidant that it was coming from the seam of the fuel pump. So come tomorrow I will return this new fuel pump and try again! I recently checked the thermal time switch which I replace a year ago and it was fine. The WUR is new. The AAR valve is new and doing it's job. All the connections on the fuse panel are fine, fuses as well! I have strong reason to believe that the front fuel pump is becoming weak. All relays are new as well. So I'm anticipating tomorrow! I will also get a can of brake cleaner and check for leaks around the throttle body. My other suspicion is that the orange o ring at the base does not seal too well.........
Old 12-25-2006, 07:01 PM
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I would check the WUR pressures, hot and cold etc. etc.
If that fails to work then a compression test is in order
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg

Last edited by NathanUK; 12-26-2006 at 07:08 AM..
Old 12-26-2006, 06:59 AM
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Try unplug the O2 sensor, see if it help.
Old 12-26-2006, 11:59 PM
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Old 12-28-2006, 05:09 PM
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So I installed the fuel pump today and that wasn't the problem! It seems to run smoother though! It really seems to be an injector issue. Tomorrow I will test out the cold start valve to be sure that it doesn't leak as well as the injectors. I'll let you all know what I find!
Old 12-28-2006, 05:13 PM
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Madkow, Im not a wizard with that 930 CIS, but have you tried pressure testing the turbo system? If there is indeed a boost leak, the mass. air door will be pulled down under spool-up and will disperse the fuel according to its position but the engine will not be getting the proper volume of air due to the boost leak. The only reason that this seems like a possibility to me is because you said the problem was under load, not during free revving.
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Old 01-01-2007, 12:36 PM
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