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'88 bucking and rough idle

I have an '88 911 Cab w/ 54k mi., she starts fine both cold and hot but after warming up just a little the idle is a little rough, nothing close to stalling but very noticable. She accelerates fine, no hesitation. The main problem is "bucking". I've done searches but found nothing like my problem. Mine bucks under no load or very light load situations, most noticeable between 2k & 3k RPM but I can feel it at other RPMs too. Creaping in 10 - 20 MPH traffic is a *****! Decelerating also seems to make it buck but nothing like it does under a very light load. If I accelerate even moderately it stops. I also have popping out the exhaust (not backfiring) under deceleration but I think that is an air leak at the muffler inlet.

I'm thinking I may have 2 (3?) problems. What symptoms besides noise do bad CV Joints have? I haven't noticed any noise from that area that I know of. Reading a few posts have me wondering if those could cause the bucking problem? Something else?? This is my main issue right now.
The rough idle I still can't figure.
The exhaust pop I'll have to seal up the leak.

I've been replacing the original vacuum lines. The rubber check valve sleeve (around PCV at oil tank vent) was the only one seriously cracked. Also replaced the Throttle Body to Air Valve vac line, both breather hoses from the oil tank, and many but not all of the smaller vac hoses.

Replaced/upgraded parts: (long before bucking started)
Steve Wong 911 Chips DME chip at 50k (updated at 52k)
New DME relay at 52k
Magnecor plug wires at 52K
O2 sensor at 50k
Dansk Euro pre-muffler & 70mm Sport Muffler at 50k
New fuel pump at 48.8k mi (leaking badly)
Full tuneup at 47.8k mi. (valve adj., plugs, cap/rotor, fuel filter, air filter ect.)

Thanks for any and all help,
Bill
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'97 Arena Red / Gray 993 Cab
'93 968 Coupe 6 Spd
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Last edited by bill3667; 06-28-2004 at 07:08 PM..
Old 06-27-2004, 07:44 PM
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Check your fuel pressures, and check your head temp sensor

AFJuvat
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Old 06-27-2004, 07:48 PM
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oddly enough, check your voltage regulator. Mine was bad and resulted in identical symptoms.
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Old 06-27-2004, 08:36 PM
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suggestions reviewed

OK, thanks.
I think I can understand the Cyl. head temp sensor and the Alt./Volt Reg. suggestions but I'm having trouble with the fuel pressure suggestion. If fuel pressure was a problem wouldn't it more likely result it hesitation on acceleration?

Seeking enlightenment...
Thanks again,
Bill W.
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Old 06-28-2004, 04:48 AM
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I too noticed that bucking problem when my CO was set too rich. It seems to have gone away when I leaned it to factory spec.

Joe
Old 06-28-2004, 09:39 AM
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This evening I checked the Cyl. head temp sensor plug and it is a single wire sensor so I'll order the new 2 wire sensor soon. Resistance was 2296 ohms (I think).
As for the battery voltage it was 12.59v w/o the car running, I'll fire it up in the next few days and check the running voltage but I suspect if the static voltage is at 12.59 the alt/voltage reg. is charging ok. This battery hasn't been on a charger in quite a while.

I'll update after I get the CHTS in and installed.

Bill W.
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'97 Arena Red / Gray 993 Cab
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'88 911 Cab Red/Blk (sold)
Old 06-28-2004, 07:07 PM
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I experienced a similar situation and the resolution ended up being the CO setting. Mine was actually running too lean.

David
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Old 06-29-2004, 07:02 AM
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the airflow meter is often the problem. find or borrow a known good one for a test drive. they only take a few minutes to swap.
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Old 06-29-2004, 07:20 AM
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Have you tried leaning the mixture just a little? I found I had the same bucking (as did stlrj ) when my mixture was too rich.

Let us know what works.

Paul
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Old 06-29-2004, 10:01 AM
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Here's an easy test to see if leaning helps. Remove the oil filler cap and take it for a drive.

Joe
Old 06-29-2004, 01:10 PM
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OK, I was hoping John Walker would chip in on this, thanks John.

I have the Bentley Manual and they have a test for the Volume Airflow Sensor(Air Flow Meter): 1st they say look for apx. 5 v. at plug pin 3, mine has 4.32v; 2nd they say between AFM terminals 2 & 3 "resistance should change without interruption", what I get is it starts at apx. 580 ohms and moving the vane causes it to fluctuate up to about 670 and back down to 580, the only linear change is between about 750 up to 1100 but then it drops down sporatically.

What I'm trying to say is if resistance is supposed to be linear as the vane is opened it's not anything close! I would suspect it needs to be somewhat linear.

So, is this test valid? If so my AFM is bad. If the test is useless I'll have to try and get a loaner. By the way, I took the AFM out of the car to test and had clips secured to the terminals using a Fluke DVM so the fluctuation wasn't the result of slipping on the test leads.

??
Thanks again,
Bill W.
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'97 Arena Red / Gray 993 Cab
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Last edited by bill3667; 06-29-2004 at 02:47 PM..
Old 06-29-2004, 02:02 PM
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The resistance of the air flow meter should be linear or at least stepped but it shouldn't go "backwards" if you know what I mean.
-Chris
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Old 06-29-2004, 03:34 PM
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You can't test an air flow meter straight with just an ohm meter. You'll need to connect a 9 volt battery across pins 3 and 4, and measure the voltage change off pin 2. As you sweep the door, the output voltage should be about 0.45 volts at full closed, and smoothly build up to the about 8 volts at full open, assuming a 9 volt input from a 9 volt battery. Carrera afms output a linear voltage, from 0 degrees to the max opening of about 95 degrees. Therefore at mid sweep of about 47 degrees, it would read 4 volts; 25% open about 2 volts, and 75% open about 6 volts. The key here is to get a smooth and consistent voltage output without any jumps or voltage gaps. If there is wear on the resistor track, it most commonly can be seen on the first 10-20 degrees or so of the AFM track. You can move the sweep arm on the shaft up or down to give the arm a new resistor track to sweep on, thus making it like new.

Also the Bentley manual is wrong about the input voltage from pin 3. The voltage comes from the DME and it is not 5 volts but in fact about 4.3 volts as you measured.

FR Wilk, the 944 chip master has posted on his site some good directions for testing and renewing an AFM here:


http://frwilk.com/944dme/afm.htm

My experience with light bucking and exhaust popping under deceleration indicates a lean part throttle mixture just off idle. This is verified with a afr monitor, and when it it richened in that region, the popping disappears. Oftentimes this is exacerbated when a free flowing exhaust is installd and the cat eliminated which leans it out in this region. Sometimes adjusting the idle mixture takes care of it. If you had bucking only, then it could be either over lean or over rich. If your afm tests out properly, I would search around for some air leaks around your intake which can cause a lean running condition. With the O2 and idle valve disconnected, use starting fluid and spray around the regions such as the intake manifold gaskets, and throttle body. If a sudden rise in the rpms is detected, you have an air leak. Intake manifold gasked often have a nasty habit of shrinking and getting sucked into the manifold tract, causing big air leaks. A couple of years ago, I once found two of my intake gaskets sucked in, and a third on its way. Also all the intake manifold nuts were loose. Usually checking and retorquing the intake manifold bolts is a maintence procedure, however be careful from overtorquing, as it can cause the flanges to warp.

Last edited by Steve W; 06-29-2004 at 06:11 PM..
Old 06-29-2004, 06:01 PM
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Thanks Steve W. I'll rig up a testing setup tomorrow after a trip to Radio Shack.
I was having a hard time understanding how the car could even run after the initial test I did.

I did post a request for a loaner AFM on our local club's web site, hopefully someone will help me out.

Bill W.
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Old 06-29-2004, 07:27 PM
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OK, thanks to Steve's AFM testing procedure I won't be spending $560 on a new one. Using < $4 in parts from Radio Shack my AFM tested fine, other than the first 6 - 8 deg. where the voltage didn't move (I suspect an idle of 800 RPM would have the vane moving more than that anyway!) the voltage output was very linear all the way to full open with no jumps or gaps. I tested it both opening and closing 2 times each and it was very smooth.

I doubt the car is running rich, I suspect it's running lean if anything. So now I'll test his vacuum leak around the intake manafold gasket theory hopefully this evening. I've already done a pretty good search for bad vac. hoses.
I'll also check the charging voltage.

I'll post the results of my tests. Thanks for all the help so far.

Bill W.
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Old 06-30-2004, 07:04 AM
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Bill, the voltage should still change during the first 6-8 degrees. After opening about 9 degrees, the voltage should double from about 0.45 volts at full closed to 0.88 volts. This could be the critical point when airflow is very low and causing it to run lean. I'd open up the black cover and move the wiper up or down to renew the meter.
Old 06-30-2004, 08:01 AM
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After doing FR Wilk's adjustment to the wiper assy. it still had the 5 - 7 deg. of no voltage change (no change to the original results). The ceramic substrate looked consistant all the way across, nothing unusual in the fully closed area.

However, I did check 5 of the allen nuts holding the intake manifold and all 5 were loose, had the wrench set to 15 Nm (they should be at 25 Nm) and all moved a good bit before reaching 15 Nm. I'm debating just going ahead and pulling the manifold and replacing the gaskets.

Bill W.
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'97 Arena Red / Gray 993 Cab
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'88 911 Cab Red/Blk (sold)
Old 06-30-2004, 06:30 PM
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Was able to test drive the car yesterday and I've eliminated about 90% of the popping back through the exhaust and 25% of the bucking problem. The rough idle problem is still there.

For the record, the intake manifold nuts are supposed to be torqued to 25Nm. I first set the wrench to 15Nm and all 12 nuts required from 1/4 to 3/4 turn just to get them to 15Nm! I then torqued all to 25Nm. I think this is what resolved the exhaust popping problem. I've ordered replacement gaskets anyway and will replace them sometime in the future (winter project?).

I had also turned the AFM mixture screw in (enrichened) 1/4 turn, I will turn it back out just to see what affect it has.

I still have to replace the original Cyl. Head Temp Sensor with the updated 2-wire style.

Steve Wong is sending me a slightly enrichened chip to try, I currently have one of his chips installed.

Bill W.
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Old 07-06-2004, 06:29 AM
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Resolution

Here is the resolution to my car's symptoms: rough idle, surging/bucking at low and no-load conditions and popping out the exhaust:

As I suspected there were 2 problems, the first was the intake manifold nuts were loose as I described in the previous post. The second problem was my 14 m.o. O2 sensor was bad. When troubleshooting I assumed the O2 was good since it had been replaced but after not being able to locate the cause of the additional problems I decided to start at the most simple possable causes and unplugged the O2 sensor, the car then ran great. I installed a new sensor but because of our heavy rains was unable to test. Test drove the car last night w/ new sensor and it ran fine, idled well, no surging and no exhaust popping!

I learned a lesson, start with the simplest possable causes first!

I did wind up upgrading to the 2-wire cyl. head temp. sensor just to prevent future problems.

Thanks for all the advice, I learned alot during this process.
Bill W.
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'88 911 Cab Red/Blk (sold)
Old 07-15-2004, 05:51 AM
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Bill3667,
I've got a similar problem with my 88 cab. I've got a really stupid question but where is the head temp sensor? I've changed the O2 sensor and got new OEM spark plug wires but I may need to change my head temp sensor.

Sorry for the stupid question but I'm still learning.....Fitz
Old 07-15-2004, 08:56 PM
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