Help/ideas please: 3.2 Carrera bucking/surging going 2500 to 3000 RPM

[Bob Prosser]

The car: Late 1980s 3.2, G-50 with totally refreshed engine 8K ago, including rebuilt distributor, Steve Wong chip, 964 cam, Fabspeed one-in, two-out. Virtually everything is new except the P’s & C’s which were deemed fine.

A week ago, the car failed a California-based STAR emissions test. The car runs very strong, and had passed no problem two and four years before. Hmm. I had my local Porsche expert go through everything related. The shop said it runs perfectly and the catalytic converter is the likely culprit. So I bought a CARB-friendly cat from D.E.C.

After a very brief two-day cat break-in, I drove the car to my Porsche-friendly emissions test shop. Two pre-check tests showed it almost passed, but it fluctuated just enough to fail the max amounts. After some discussion, I decided to run an official test anyway. Note: the car was idling the entire time while the shop proceeded the testing sequence, etc. During this time the engine temp gauge had origionally been hovering at the normal 200-220F mark, but by the time we were ready to test, idling inside the shop, the temp was up to about 250.

Results: the car passed with very good readings. Apparently, the cat needed to get HOT to do its job. Values when passing emissions:



Now to the problem: Post-test, I drove the car home 15 miles. The car ran GREAT until the last two stop lights, i.e., when the light went green, in 1st gear, it started bucking/surging as it climbed through the 2500-3000 RPM rev range. Oddly, it did not do that when in 2nd, 3rd, etc. I figured the problem was just a by-product of the hot test session and would not happen again once things cooled off.

Next day: the car started, warmed and idled fine. However, now it bucks/surges in every gear as it climbs through that particular rev range of 2500-3000 RPM. After that it revs well. Seems a repeatable problem that is getting worse.

Idea on culprits?
• Bad fuel filter? (might be 40K miles old)
• DME cold solder joint?
• Head temp sensor? (update added 90K miles ago)

The O2 sensor is 8K mi old, and apparently works well. I hate to just start throwing parts at it.

Is the climb of 2500 to 3000 RPM a clue?

Thank you for your thoughts!

[proporsche]

Hi Bob i`d start with the basic..the filter for sure 40 K is a lot of miles.When you replace it save the fuel in a glass jar and se..then report..In LA i used to change the filter every year due to the different gas stations and quality of gas..

Ivan

[Bob Prosser]

Ivan, thanks for the reply.

Yes the fuel filter might be overdue. I will have a new one on soon. But I'm wondering why the problem started immediately after the smog test with the hot engine ... and why problem (right now anyway) is only being exhibited between 2500 and 3000 RPM?

[proporsche]

It could be the temp head sensor but you can do the test if you like before you get the fuel filter look here..but start replacing the fuel filter 1st...is your oxy sensor plugged in ?If yes disconnect it and drive it..The oxy sensor is plugged on the left side of the engine right under the fuel filter
Another thing could be it is too lean ..i would add some fuel to it ,do you know how?



or eventually testing the other sensors....

[darrin]

wondering whether proporsche's "lean running" comment could be tied to a developing air leak somewhere in the intake system (downstream of the afm/throttle body) that's letting in (progressively more) unmetered air and leaning out the engine -- also interested in hearing what happens if the o2 sensor's disconnected --

[GH85Carrera]

I have twice had a similar problem. Weird surging like that.

Once was a bad coil. Easy fix.

The second time was a bad speed sensor on the flywheel. I replaced both sensors and the issue was fixed.

[Bob Prosser]

Gentlemen, thank you for your input so far.

Regarding the lean comment, the engine had been out only a thousand miles ago and we put a better throttle body and micro switch on it. Everything was checked.

The coil is original to the car so that could be an issue I suppose.

I will check the head temp sensor and flywheel sensor as soon as possible.

I'm wondering what the climb of 2500 to 3000 RPM using moderate throttle has to do with it. Is there a transition in the fuel mapping or something?

[proporsche]

Bob i`d start with the filter and richen it up because the 0,08 is really very lean you should go to 1,00-1,5 at least without the oxy sensor plugged in...

[Pedro '84 Coupe]

I had a similar issue on mine, although it's completely stock.

Mine was a combination of old fuel filter, bad CHT sensor (replaced the flywheel sensors too since the connectors were brittle) and the track on the AFM was worn. I think repositioning the wiper on the AFM did the trick though because throttle response was so much better. I also replaced the intake manifold gaskets at the same time. Did the O2 sensor and connector too. Since your intake gaskets are relatively recent, you may want to check the torque on the fasteners as they can get loose over time. A long hex socket with the ball end made that easy.

After all that I had the base idle and CO2 set and it runs great. Good luck!

Edit: Sweet looking car, btw!

[Bob Prosser]

Friends, so, I just got done replacing the fuel filter. I don't think it was the problem since apparently my shop changed it without letting me know when the engine was rebuilt. Once I yanked it, I could see the Mahle mfg date was 2/2015. That, and there was no dirt seen when I poured fuel out into the pan.

However, while I was in there, and the heater fan duct was off, I checked the connections for the head temperature sensor, etc., and noticed that it was slightly yellowed and looked a little wonky in its fit. So, I unplugged / snugged everything up a few times, gave all connections a few "love taps" and took it for a spin.

ZERO symptoms!

I'm guessing the excess heat in the engine bay during the emissions test may have stressed the connections. I'm hopeful this is the end of the story. We'll find out!

Thank you very much for all your input.

[proporsche]

Good job BOB;-) if it was MFD on the filter -old stock- i hope you replaced it anyway,right?

Ivan

[Pigeye]

You might check this thread. Had similar issues. Replaced both flywheel sensors, head temp sensor, fuel filter. Turned out it was the coil.

https://forums.pelicanparts.com/porsche-911-technical-forum/1039995-3-2-running-like-has-hair-ball-stuck-throat.html

[proporsche]

Hey Bob, did you ever go to your namesake street in Hollywood;-)??
Ivan

[Bob Prosser]

Originally Posted by proporsche

Hey Bob, did you ever go to your namesake street in Hollywood;-)??
Ivan

Sure have!

[proporsche]

i`m sure it must be a cool feeling;-) i have found one in Italy-Lago De Garda, last year just the 1st name thou...

[Bob Prosser]

Update: with a newly acquired air/fuel ratio meter, I was able to get the AFR numbers to where I needed them. They now are:

• AFR at idle: 14.4± (desired 14.4)
• AFR at 3K RPM in 3rd gear: 14.2 - 14.6 (desired 14.2 - 14.7)
• AFR at 3K RPM in 4th gear: 14.2 - 14.7 (desired 14.2 - 14.7)

Apparently, the car was running a bit too lean before, but unfortunately the stumble was still there from time to time. Dang it.

On a hunch, I pulled the distributor cap to inspect the distributor itself, having been rebuilt two years ago. When I removed the rotor, I noticed it wasn't very snug on the shaft meaning there was a lot of extra movement going counterclockwise when pushed. (This video will demonstrate: https://youtu.be/PzhR354Ij4c.) I measured the movement. It could move a full 10° CCW.

I bought a new rotor from Porsche (PN 930-602-902-01) and compared it side-by-side to the old one. All the numbers were the same, except the new one had a "2.3" embossed underneath, whereas the used one had "2.4." See pix below. Anyone know why there's a difference? The new rotor fit nice and snug, and had almost none of the movement you see in the video.

I buttoned everything up, started the car and drove it for over 30 miles until fully warm in every conceivable condition, including stop-and-go traffic. The car drove perfectly! ZERO stumble at low RPM as before. I write this in hopes others who have been chasing this problem (and there are many) might give this a try. It's a cheap & easy fix. Thanks.

[proporsche]

very very interesting i`ll look at my stock manana what it says..glad you got it fixed...good info

Ivan

[Tea Tray]

You might consider going back to make sure all three sensor plugs and connectors are in good shape.
I’ve found connections damaged by heavy handed test probing causing the female socket to spread - causing an intermittent connection.
Once good connections are confirmed, consider dielectric grease on the connections.

[911obgyn]

I second the dielectric grease, damp conditions cause corrosion and the grease keeps the damp out!

[Bob Prosser]

Tea Tray and 911obgyn: In the past, I have gone over all the connections and they are fine, though that's a good idea adding the dielectric grease since I live very near the Pacific Ocean. Thanks.