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By Simon928
#76435
Hi everyone! Since doing a complete intake refresh on my '90 S4, I have been experiencing an issue with RPM surging that only manifests itself on deceleration. The car idles fine, but when I'm driving 40+ mph and take my foot off the throttle the RPMs drop to 1100 and then bounce up a few hundred RPMs, then drop down to 1100, then the process repeats itself until I slow down enough to break through that 1100 RPM barrier.

Things I have tested:

Vacuum System
I have done multiple smoke tests on the intake with nary a wisp of smoke escaping anywhere. I have also checked the vacuum at the front damper when the engine is running with my MityVac and it is around 16" Hg. I have read before that it should be around 18" Hg--is that 2" difference enough that I should be looking into it or could it just be variance in measuring devices?

Fuel System

My FPR and rear damper didn't seem to be leaking fuel but they didn't hold vacuum either so I replaced them. No effect on the RPM issue.

TPS
I confirmed that the computer is getting the signal from the TPS by testing with my multimeter at the EZK as well as by confirming that my digidash showed 90 MPG when I took my foot off of the gas after driving on the highway.

Is there anywhere else that I should be looking that could be causing this RPM issue? O2 sensor? MAF? LH? I have seen some mentions of a rich mixture causing surging but I was wondering if the fact that this only happens on deceleration isolates things or not.
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By worf
#76441
Check for idle switch activation at ECU connector pins for *both* LH and EZK.

At same time check ISV function at LH pins.

Have you made intake/crank case breather modifications?

When were injectors last cleaned/balanced?

O2 sensor and MAS history?

EDIT: does the engine idle at Spec?
amdavid, MFranke liked this
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By Simon928
#76494
Sure, I'll check the signal for both the idle switch and the ISV at the LH pins as well. Answers to your other questions are below:

Have you made intake/crank case breather modifications?
I replaced all of the vacuum lines and breather hoses but I kept all of the original routing.

When were the injectors last cleaned/balanced?
I didn't send them out to be cleaned as the only place to send it to was in the U.S. (I'm in Canada) and the post service was slowed down to a snarl when I was doing my intake refresh late last year. If the problem could be associated with that I could certainly look into it now as mail speeds seem to be more or less back to normal.

O2 Sensor and MAF history?
I've had the car since 2016 and they haven't been replaced on my watch. Not sure about before my ownership.

Does the engine idle at spec?

It idles steady but at around 800 RPM which I think is a bit high.
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By worf
#76836
Simon928 wrote: Wed Apr 28, 2021 4:17 pm Sure, I'll check the signal for both the idle switch and the ISV at the LH pins as well. Answers to your other questions are below:
The main thing is to check the ISV, but while you're there go ahead and check-off that the idle switch signal makes it to the LH.

Simon928 wrote: Wed Apr 28, 2021 4:17 pm I replaced all of the vacuum lines and breather hoses but I kept all of the original routing.
Ok. And the smoke test was negative. So, not air related.

Leaves fuel and control system.

You tested both dampers and the fuel pressure regulator to make sure that they hold vacuum? (... that the diaphragms haven't ruptured?)


When were the injectors last cleaned/balanced?
Simon928 wrote: Wed Apr 28, 2021 4:17 pm I didn't send them out to be cleaned as the only place to send it to was in the U.S. (I'm in Canada) and the post service was slowed down to a snarl when I was doing my intake refresh late last year. If the problem could be associated with that I could certainly look into it now as mail speeds seem to be more or less back to normal.
A leaky injector could lead to surging on overrun. Testing/cleaning facilities usually test for that before the service.

If you have an external fuel pressure you can test for leakage.

O2 Sensor and MAF history?
Simon928 wrote: Wed Apr 28, 2021 4:17 pm I've had the car since 2016 and they haven't been replaced on my watch. Not sure about before my ownership.
Mass-Air Sensors are typically out of spec after 60k miles. Deterioration depends upon heat cycles not mileage. If you have no history on your MAS, sending it out for a check is a good idea. Rich Andrade at https://electronikrepair.com/ can test the MAS.

Does the engine idle at spec?
Simon928 wrote: Wed Apr 28, 2021 4:17 pm It idles steady but at around 800 RPM which I think is a bit high.
S4 should be 675 +/- 50 once warm. It will idle higher until warm. A slightly high idle is another indication of possible excess fuel.

Have you tested TEMP-II resistance values at ECU pins? (Resistance table in my inspection guide.)

What's the history on the LH ECU? Has it been rebuilt yet?
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By Simon928
#76969
Thanks for the further suggestions! I am hoping that I will have a bit of time to do some further testing this weekend. We're under lockdown where I live and we can't go anywhere so I'm spending all of my time trying to entertain my two little kids, so my time to work on the car has been scarce. :)

One question: When you said "If you have an external fuel pressure you can test for leakage", are you referring to one of those fuel pressure gauges that attaches to the fuel rail? If so I don't have one but I could certainly order one online.

A few answers to your questions:

Yes, I did check the new FPR and new rear FPD and confirmed that they both hold vacuum. My old front damper also holds vacuum.

I have not tested the Temp II sensor, though that is one of the items I replaced in my refresh. I'll add that to the list of things to check this weekend.

My LH is original and hasn't been rebuilt as far as I am aware.
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By worf
#77078
While there are a number of possibilities, based upon what I think I know about your 928, I would send the MAS out for testing before spending money or non-trivial time on other diagnostics.

Simon928 wrote: Fri Apr 30, 2021 1:57 pm One question: When you said "If you have an external fuel pressure you can test for leakage", are you referring to one of those fuel pressure gauges that attaches to the fuel rail? If so I don't have one but I could certainly order one online.
Yup. You need a new fuel rail cap nut also. Drill and tap the cap nut for the same threads as the FP gauge. Seal the threads with a fuel-friendly thread sealer. Use a block-drain crush ring between the cap nut and fuel rail.

Good for testing only.

This, only if you suspect that you've a leaky injector.

Simon928 wrote: Fri Apr 30, 2021 1:57 pm I have not tested the Temp II sensor, though that is one of the items I replaced in my refresh. I'll add that to the list of things to check this weekend.
It's not uncommon for folks to replace a TEMP-II and have TEMP-II issues because the sender was never the problem. The problem was high-resistance at connectors or at water bridge.

The TEMP-II grounds through the water bridge to the block. If you've a poor ground, flakiness can happen.

When you did your intake refresh did you do any kind of surface coating on the water bridge (e.g. paint, etc.?)

Simon928 wrote: Fri Apr 30, 2021 1:57 pm My LH is original and hasn't been rebuilt as far as I am aware.
Well... that might also be a problem but since the 928 still runs, it's hard to test. If there's another 87+ in the vicinity...

One other thing, when you re-check the idle switch signal at the ECU pins, make sure to test several times with different 'pedal movement.' Examples: abrupt transition from full throttle to idle, very slow transition to idle, etc.

A couple of times I've found idle switches set up such that the always worked with abrupt movement but wouldn't always work with 'gentle' movement.
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By Simon928
#77656
I was finally able to find a few minutes to do some of the diagnostics that you suggested. First off, thanks so much for making that inspection guide! It's an awesome resource that I can see myself using a lot in the future.

Regarding the multimeter diagnostics I was able to do most of them but I do have a question. When I was looking at your guide for doing the Temp II test, I was able to do it for the EZK but not for the LH. Your diagram and associated instructions are for 87-88 cars but mine is a 90 and in the pin out diagrams for my LH I don't see a pin specifically for the Temp II. Do you know which pin I should be using for the test? For the Temp II EZK the result was 3.4 (with the engine cold), which seems to be a bit lower than the range that you said it should be.

Image

I also tested the ISV at the LH and I heard the click when I jumped the pins, so it seems to be working.

Just for the heck of it I also tested my TPS both at the computers and at the switch itself and I'm getting the expected readings. Like I mentioned earlier I also get the 90 mpg reading on my digidash when I take my foot off the gas on the highway so that makes me think the TPS probably isn't the issue.

Also regarding your question about my water bridge, I didn't put any paint on it during my refresh.

Unless there is a way I can test my Temp II at the LH I might put my old one back in there and see if it makes a difference. If not I will send out my Mass Air Flow sensor to get rebuilt.
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By worf
#77667
Simon928 wrote: Mon May 03, 2021 10:57 pm First off, thanks so much for making that inspection guide! It's an awesome resource that I can see myself using a lot in the future.
Thanks!

Simon928 wrote: Mon May 03, 2021 10:57 pm Regarding the multimeter diagnostics I was able to do most of them but I do have a question. When I was looking at your guide for doing the Temp II test, I was able to do it for the EZK but not for the LH. Your diagram and associated instructions are for 87-88 cars..
Nope. My instructions are good for all LH 2.3 and my connector diagram is not *exactly* like that from the troubleshooting guide that you posted above. There is a subtle difference.

Simon928 wrote: Mon May 03, 2021 10:57 pm ... but mine is a 90 and in the pin out diagrams for my LH I don't see a pin specifically for the Temp II. Do you know which pin I should be using for the test? For the Temp II EZK the result was 3.4 (with the engine cold), which seems to be a bit lower than the range that you said it should be.
Pin 13: NTC-II

The pins are the same for '87-95. However, starting in '90 the LH connector is reversed. That's why *my* connector diagram is different from "the book." See the difference? I've marked one end '87-89 and the other '90-95.

Image

3.4 k-Ohms would correspond to a core temperature of about 75-ish °F.

Simon928 wrote: Mon May 03, 2021 10:57 pm I also tested the ISV at the LH and I heard the click when I jumped the pins, so it seems to be working.
You should hear two clicks: one when you ground the pin and one when you remove ground. The clicks should have no lag and be nice and 'sharp' sounding.

Simon928 wrote: Mon May 03, 2021 10:57 pm Just for the heck of it I also tested my TPS both at the computers and at the switch itself ...
Did you try it fast, slow, etc.?

Simon928 wrote: Mon May 03, 2021 10:57 pm Also regarding your question about my water bridge, I didn't put any paint on it during my refresh.
Good then you should have no ground problem for the TEMP-II (NTC-II).

Simon928 wrote: Mon May 03, 2021 10:57 pm Unless there is a way I can test my Temp II at the LH I might put my old one back in there and see if it makes a difference.
See above.

Simon928 wrote: Mon May 03, 2021 10:57 pm If not I will send out my Mass Air Flow sensor to get rebuilt.
Yup. Rich Andrade can test it. He'll let you know 'where it's at' calibration wise and then you can decide one way other the other.
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By Simon928
#77678
Ahh okay, that makes sense. I just saw somewhere that 90 and newer cars had different plugs but I didn't realized that they were just reversed. I'll check the Temp II at the LH at my next opportunity. Thanks again for all of your help!
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