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Super Jenius
 
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930 Hesitation/Stutter

Gentlemen, I need your help.

The situation is a 1979 930, twin plugs, Electromotive ignition, TiAL wastegate and Kokeln intercooler/exhaust.

Car starts immediately and runs strong; however, as we get to the 4000 rpm area, it feels like it loses a bit of power, and then... stutters or judders a bit. If you put your foot into it, it will come right out of this bouncy/jerky zone, but you can't cruise at 4000 rpm area b/c it's uncomfortable. Once you're above about 4500, it goes away, so it's not like an increasingly bad judder cause by bad wires or what have you.

This is not Turbo lag. I start to get boost at about 2800 rpm, and I know what lag feels like. This is something else. It kind of *feels like* fuel is stopping and starting (or being cut out and in) a couple of times a second. I have a HALmeter on the car, and it doesn't show a change in lambda when this happens (but it might not be "fast" enough to detect these pulses).

What I've done (my mechanic has done, mostly) thus far:

Checked all 12 plugs -- they look good.
Checked both fuel pumps -- both have good pressure and flow
Changed fuel filter
Changed fuel line to distributor (it was worn and weak)
Generally tried to tune the car... it idles solid and, other than the 4000 rpm stutter, runs well.

So... a few thoughts/guesses?
The boost pressure sender is not working (the dash gauge stays pegged at 1 when the car is on -- I have a separate boost gauge/controller that I use. Could the boost pressure sender's failure cause such a problem? Is there some sort of integral fuel cut-out that the pressure sender might be causing?

There is another sender "next to" the boost pressure sender, whose function is a mystery to me. If anybody can enlighten me as to that little doohickey's purpose, and what problems it might cause, I'd appreciate it.

Any other ideas/theories would be greatly appreciated. This is the last "issue"with this car, and I'd enjoy it a whole lot more if I could get rid of this stutter/judder.

As always, thanks in advance.

Best,

JP

Note -- also posted in the 911 Tech Forum.
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2003 SuperCharged Frontier ../.. 1979 930 ../.. 1989 BMW 325iX ../.. 1988 BMW M5 ../.. 1973 BMW 2002 ../..1969 Alfa Boattail Spyder ../.. 1961 Morris Mini Cooper ../..2002 Aprilia RSV Mille ../.. 1985 Moto Guzzi LMIII cafe ../.. 2005 Kawasaki Brute Force 750
Old 06-17-2010, 02:17 PM
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my '86 was doing just that. I unplugged the o2 sensor and it ran beautifully. Not sure your '79 has this.
Old 06-17-2010, 02:59 PM
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#1The boost pressure sender won't cause poor running, and won't cause fuel cutoff.
#2Fuel cutoff is quite obvious and dramatic engine shutdown. Fuel starvation would be a separate issue, but I doubt that's your issue.
#3Not sure what that other doohicky sender next to the boost pressure sender is. Picture would help.

My guess....check for an air leak in all the plumbing downstream from the turbo. If you're loosing boosted air, you'll get progressively richer and could cause the stumble/lack of power. Could be other stuff like a bad vacuum connection somewhere, or ignition timing dropping off.
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 06-17-2010, 03:54 PM
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Few things;
What plug gaps are you running?


On the Electromotive system, what are your settings? What is your rev-limiter set at ? Are you running XDi or the older HPV-1 system? Are you running a MAP sensor or vacuum sensor (older HPV-1 systems)?
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'85 930 Factory Special Wishes Flachbau
Werk I Zuffenhausen 3.3l/330BHP Engine with Sonderwunsch Cams, FabSpeed Headers, Kokeln IC, Twin Plugged Electromotive Crankfire, Tial Wastegate(0.8 Bar), K27 Hybrid Turbo, Ruf Twin-tip Muffler, Fikse FM-5's 8&10x17, 8:41 R&P

Last edited by WERK I; 06-17-2010 at 05:39 PM..
Old 06-17-2010, 05:37 PM
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Turbofrog
 
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Quote:
Originally Posted by Overpaid Slacker View Post
There is another sender "next to" the boost pressure sender, whose function is a mystery to me. If anybody can enlighten me as to that little doohickey's purpose, and what problems it might cause, I'd appreciate it.
That could be the over boost switch/sensor?

It would be nice to know the airfuel ratios while you have the spuddering. Quite possibly the boost starts to build up at those rpms even just cruising and that might have an affect on some of the hoses/IC connections. If rpms higher the need fo gas is higher so the extra richness of boost leak might rule out?
Old 06-17-2010, 09:53 PM
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Try to ground the cable plugged into the boost switch. If your car feels good now, replace the boost switch.
had this problem with mine.
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1979 930: Garret GT35r turbo, EFI, carerra intake, Link EMS, custom GT2 cams, 98mm JE P/C, 964 crank (stroker), custom valves & ported (XtremeCylinderHeads) etc..etc..
1972 914-6 GT replica project
1986 944 Turbo
Old 06-18-2010, 06:43 AM
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Super Jenius
 
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First of all, thanks for the feedback.

Will try unplugging the O2 sensor.
The overboost protection cuts fuel? That may be the culprit as well. If it's malfunctioning and cutting fuel sporadically at a certain rpm.... that could be it.

Re: Electromotive, it's old. TEC-3, I think. It's pretty much "set it and forget it" AFAIC. I don't think the settings are an issue. I'm 90% it's not the ignition map. The issue clears up once you're through the 4000-4500 rpm range. It's just a huge PITA to try to cruise around 4000.

The air/fuel stays pretty much at 14:1, going to about 11:1 under full enrichment/boost. Again, the HALmeter might not be the quickest to respond, but if you're just moving down the road @ 4000 rpm, you get the judder, but the ratio is still @ 14:1.

Will try your suggestions over the weekend and report back.

drmatera -- out of curiosity, what made you think to unplug the 02?

JP
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2003 SuperCharged Frontier ../.. 1979 930 ../.. 1989 BMW 325iX ../.. 1988 BMW M5 ../.. 1973 BMW 2002 ../..1969 Alfa Boattail Spyder ../.. 1961 Morris Mini Cooper ../..2002 Aprilia RSV Mille ../.. 1985 Moto Guzzi LMIII cafe ../.. 2005 Kawasaki Brute Force 750
Old 06-18-2010, 06:47 AM
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pelican made me do it. I just did a search on stumble 4000rpm and noticed alot of guys metioned the o2 sensor could be bad and a quick way to check it was to just unplug it. I did, and the car ran 99% better. As the rpm's hit 4000rpm it would cough, sputter and generally make NO power. Then if you stay with it and fight thru it would start to clear around 4500rpm then take off like a rocket. It was worse when cold but still did it warmed up. The last 1% improvement was disconnecting the huge gold vacuum diaghram under the intercooler. That seemed to cure the last little bit of jerkyness the car had.
Old 06-18-2010, 07:52 AM
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OFF THE BOOST PIPE NOW...
 
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I have a friend running a TEC-3 without problems. It's not a plug and play system, as some work is needed to dial it in right. Have you had it dynoed with a AF map?
Old 06-18-2010, 08:09 AM
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Had a similar situation...Drove me NUTS!!!!!!!!!!!!!!
CHk -all air connections.
Chk -all ignition contacts--placed on tight.
CHK-your gap.
CHK-your plugs again---I had a hairline fracture in one/hard to see--That was my problem.

Then changed to denso's w24esr-u gap. .032-stock--Never had a problem since then

Walt
Old 06-18-2010, 09:12 AM
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Super Jenius
 
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The TEC-3 was dialed in a few years ago by some Electromotive guru, and I haven't futzed with it since. I haven't dynoed it.

I'll re-re check all of the air connections and ignition contacts, but this'll be the 3rd time. Just re-gapped plugs 2 days ago.

Being twin-plugged, I don't know if a single bad plug would cause this degree of drama, but it's worth checking.

JP
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2003 SuperCharged Frontier ../.. 1979 930 ../.. 1989 BMW 325iX ../.. 1988 BMW M5 ../.. 1973 BMW 2002 ../..1969 Alfa Boattail Spyder ../.. 1961 Morris Mini Cooper ../..2002 Aprilia RSV Mille ../.. 1985 Moto Guzzi LMIII cafe ../.. 2005 Kawasaki Brute Force 750
Old 06-18-2010, 09:29 AM
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Tec-3 is Electramotive electronic fuel injection and ignition from around 5 years ago.
From your description it sounds like it's running lean in the steady cruise midrange and needs adjustment.

The oxygen sensor removal mentioned before is from the 1986 and later USA version CIS fuel injection lambda emission system and doesn't apply to your Tec-3 EFI.

The Halmeter is an old and obsolete narrow band AFR guage kit that has a very limited innacurate range and is totally useless for tuning a car.

You need a new design quality wideband AFR kit like an Innovate system to know what your AFR's really are or a good dyno with accurate AFR logging.
Old 06-18-2010, 10:51 AM
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Super Jenius
 
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Thanks.
I assumed the 02 sensor was not going to be the answer on this vintage of a car, but it's costless to disconnect it and go for a ride...

The Electromotive is running the ignition only; not the fuel. I updated the software/system about 3 years ago to the then-current state of the art software, but the system itself is probably 10 years old(?)

The LM2 would be a nice addition, but it's a bit spendy.

At this point, the mechanic tells me he's dialed the judder out at cruise, it's only a WOT that there's a pause transitioning through 4000 rpm.

JP
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2003 SuperCharged Frontier ../.. 1979 930 ../.. 1989 BMW 325iX ../.. 1988 BMW M5 ../.. 1973 BMW 2002 ../..1969 Alfa Boattail Spyder ../.. 1961 Morris Mini Cooper ../..2002 Aprilia RSV Mille ../.. 1985 Moto Guzzi LMIII cafe ../.. 2005 Kawasaki Brute Force 750
Old 06-18-2010, 11:05 AM
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elvision930
 
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I just went through this very same issue. Bothered me for several months.

Mine however studdered between 2000 - 3000 RPM until the boost kicked in. then fine. But cruising around town in normal driving situations was quite frustrating. I then changed my plugs, checked the gaps, and changed my O2 sensor. I also noticed my lambda was not securely bolted down. So I sandpapered the ground, bolted her down and checked all connections. One connection under the CDI unit was not quite on. Spread the gaps on my yellow boost sensor as well to make a snug fit.

Never the less, what ever I did.... so far that particular problem seems to be behaving itself.
No hesitation or studder as you describe. Hope that helps. Knock on wood.
Old 06-18-2010, 11:06 AM
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Quote:
Originally Posted by Overpaid Slacker View Post
Thanks.
I assumed the 02 sensor was not going to be the answer on this vintage of a car, but it's costless to disconnect it and go for a ride...

The Electromotive is running the ignition only; not the fuel. I updated the software/system about 3 years ago to the then-current state of the art software, but the system itself is probably 10 years old(?)

The LM2 would be a nice addition, but it's a bit spendy.

At this point, the mechanic tells me he's dialed the judder out at cruise, it's only a WOT that there's a pause transitioning through 4000 rpm.

JP
OK, I assumed you had EFI if you have Electramotive TEC-3 in your car.

If your is a 1979 model and is running the origonal CIS injection it still has no oxygen sensor.
The lambda system with O2 sensor did not show up on a 930 until the 1986 USA models came out.

The innovate LC-1 wideband kit is less expensive than the LM-2 and just as accurate with AFR readings. Buying and installing one yourself is ALOT less expensive than paying someone else to figure it out or do multiple dyno runs.
Old 06-18-2010, 11:59 AM
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