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GTS stutters/buckles at idle and when giving quick full throttle.

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GTS stutters/buckles at idle and when giving quick full throttle.

 
Old 07-23-2018, 08:53 PM
  #46  
Arnoud
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Default First results of my intake refresh: idle good, but stutters/buckles still there...

After initial start and doing the idle adaptation with help of Theo's tool, and having checked for any vacuum leaks by spraying brake cleaner around all rubber seals (and happily to report: no single vacuum leak found), these are the results:

1. Idle is now good, no dropping rpm's to 500 anymore. So the intake refresh and having changed literary everything has solved that problem (I personally think it was the old ISV, as it was full with 24 years of gunk inside it - even though it was still working "on the bench" OK).
2. But the main problem of it stuttering/buckles and misfiring under full trottle from idle is still there (all works fine when giving "gas" slowly). Oh crap!

I checked the spark plugs, after several hours of testing at stand still - i.e. NOT yet having driven the car around: they seem to all indicate that it is rich/very rich when applying full throttle:

Other things as tested:
- I did a knock count test with Theo's tool after having putting the spark plugs back in, while changing the revs between idle - 2000 rpm: 0 in 10000 ignition events. So all appears to be good with that too.
- And no LH nor EZK nor any other electronic module faults found either.
- MAF swapped with another unit: same problem remains.

Anybody having any idea what problem I must be having (as I have literary changed everything during this intake refresh, apart form the Hall sensor and the final stages. But if the Hall sensor would be malfunctioning, then how on earth can all be working fine when giving full throttle slowly... And clearly I am having sparks on both banks as well, so I guess there is nothing wrong with the final stages either)???

Last edited by Arnoud; 07-23-2018 at 09:18 PM. Reason: More info added.
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Old 07-24-2018, 04:52 AM
  #47  
FredR
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Arnoud,

Plug No2 looks a bit suspect - have you had the fuel injectors tested for spray pattern/fuel delivery etc? My experience with the Bosch injectors suggests they need cleaning every 10 years or so.

Running the engine under "no load" conditions is not going to tell you much [or so I would think]. On full load the stock tuning runs a bit on the rich side- typically about AFR 12.4 but that is a bit academic as you have not had the motor on full load . Not sure I would want to run a torque converter at full rpm's in the park position- perhaps a few full throttle blips when fully warmed up but no more than that. I suspect you need to get the car on the road or on a dyno at least to form any meaningful conclusions.

Rgds

Fred
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Old 07-24-2018, 06:06 AM
  #48  
Arnoud
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Originally Posted by FredR View Post
Arnoud,

Plug No2 looks a bit suspect - have you had the fuel injectors tested for spray pattern/fuel delivery etc? My experience with the Bosch injectors suggests they need cleaning every 10 years or so.

Running the engine under "no load" conditions is not going to tell you much [or so I would think]. On full load the stock tuning runs a bit on the rich side- typically about AFR 12.4 but that is a bit academic as you have not had the motor on full load . Not sure I would want to run a torque converter at full rpm's in the park position- perhaps a few full throttle blips when fully warmed up but no more than that. I suspect you need to get the car on the road or on a dyno at least to form any meaningful conclusions.

Rgds

Fred
Hi Fred,

Thank you for your prompt follow up comments and question.

All eight fuel injectors were ultrasonic cleaned, had their (micro)filter changed + new pintle cap + "washer" + top- and bottom-O-rings (both correct size for our cars). Then they were then tested on induction rating + maximum flow capacity + dynamic flow test @3mS:
Injector #: Induction, mH: Maximum flow, cc: Dynamic test @3mS, ml:
1 11,5 200 101
2 11,4 202 100
3 11,3 202 104
4 11,7 202 101
5 11,2 202 103
6 11,7 200 103
7 11,5 202 100
8 11,3 198 103
Deviation: 2% 4%

So injector testing wise: all looks to be good to me.

Next step then for me is to get the car on the road, disconnect and reconnect the battery and then drive around so to have the computers learn the fueling settings. I will report back here after that.
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Old 07-24-2018, 07:16 AM
  #49  
FredR
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Arnoud,

The test values look fine but when was this test carried out? For reasons I know not, if the injectors are allowed to sit for a period of time this strangely enough can cause issues. When we were preparing my current 928 I had two sets of injectors- those from my S4 motor that we put in the GTS chassis and the original GTS injectors. I had my S4 injectors cleaned and worked perfectly, they sat for about a month and upon start up of the project I could feel something was obviously wrong. We pulled the injector connectors one at a time and found one of them was not working- stuck a spare from the GTS set in and problem solved. Not saying something like this has happened but just because you believe with reasonable logic that the injectors are OK does not mean that is necessarily so.

You might consider taking a look at the piston crown on number 2 if you have access to an endoscope. Presumably your distributor cover and rotor arms are in good condition/reasonably new? You can also try running the motor in a dark environment and look for any "spark leakage"- presumably all your HT leads are in good condition and not 24 years old.
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Old 07-25-2018, 10:38 AM
  #50  
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agree with Fred, #2 looks bad.........those are not the correct spark plugs!

change the plugs and do a compression check/leakdown check.

You need to drive the car to get it to full operating temp then look at plugs etc.
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Old 08-10-2018, 02:07 PM
  #51  
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Originally Posted by FredR View Post
Arnoud,

The test values look fine but when was this test carried out? For reasons I know not, if the injectors are allowed to sit for a period of time this strangely enough can cause issues. When we were preparing my current 928 I had two sets of injectors- those from my S4 motor that we put in the GTS chassis and the original GTS injectors. I had my S4 injectors cleaned and worked perfectly, they sat for about a month and upon start up of the project I could feel something was obviously wrong. We pulled the injector connectors one at a time and found one of them was not working- stuck a spare from the GTS set in and problem solved. Not saying something like this has happened but just because you believe with reasonable logic that the injectors are OK does not mean that is necessarily so.

You might consider taking a look at the piston crown on number 2 if you have access to an endoscope. Presumably your distributor cover and rotor arms are in good condition/reasonably new? You can also try running the motor in a dark environment and look for any "spark leakage"- presumably all your HT leads are in good condition and not 24 years old.
Thanks Fred,

For your continued help and thinking along, which helps me so to logically eliminate the likely problems so that in the end (sooner rather than later, I hope) we'll find the root cause.
And yes: the distributor covers and rotor arms and coils are all in excellent conditions, as per the first photo's and posts in this thread (from over 1 year ago now ). All those and the HT leads were all renewed by me as preventive maintenance during my year 2014 TB + WP job. No "spark leakage" observed when I tested for that in the dark during the last few days, unfortunately.
I have done a bunch of tests thus far, which I will now first document here. Then I still have a few tests left to do, including your good suggestion for testing the injectors by pulling them one by one and observing the idle speed.

Originally Posted by the flyin' scotsman View Post
agree with Fred, #2 looks bad.........those are not the correct spark plugs!

change the plugs and do a compression check/leakdown check.

You need to drive the car to get it to full operating temp then look at plugs etc.
Thanks for your help, Malcolm!

Yes: these spark plugs of mine do not look too good anymore, I agree. These Bosch WR7DTC spark plugs are the recommended spark plugs for our GTS cars, but maybe that was more marketing at the time (mid 1990's) then really being something special or mandatory... I did follow up on your advice on changing the plugs and doing a compression test, among checking and testing many other things thus far - which I will now first document here.
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Old 08-10-2018, 02:22 PM
  #52  
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Arnoud,

If there was not a problem with your 928 you would be bored!

The beauty of these cars is the problems surface quicker than they are fixed- it is a more socially acceptable equivalent to sado masochism or so I suspect!
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Old 08-10-2018, 03:20 PM
  #53  
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As my problems from over 1+ year ago, for which I parked my car permanently into the garage and before I started to do a full intake refresh so to hopefully fix this (which is has not), still remain: I therefore need all the help that I can get! Hereby the results of tests I have done in the past few weeks, with as main problems still:
i. When at idle, the engine RPM's drop down every 10 seconds or so: from around 675 to around 500 for about 1 second (I was thinking that it was solved now, but it has come back).
ii. When giving quickly full gas/throttle from idle to around 2000 RPM, it stutters/buckles completely - including misfire/loud pops. If instead slowly revving the engine from idle to around 2000 RPM: all works - sort of - fine.

Additional tests done now:
1. Measured all spark plug leads (around 5K ohms each), and put DeOxit on both ends of the leads. These are all new correct Beru leads from the year 2014, which I then put in as prevented maintenance for the then existing Beru leads (of which age I was not sure about - most likely those were the original ones still).

2. Checked for "spark leakage / fire flies" with engine at idle at dark: none observed anywhere.

3. Used two (2) new 2nd stage ignition MOSFET modules: no change, problems still there. Put the originals back, with new fresh white paste (as used for cooling CPU's on heatsinks).

4. Capped the y-rubber hose from the venturi to brake booster, and the brake booster: no change, problems still there.

5. Replaced the venturi tube o-ring, with a new 10mm * 2mm o-ring. This makes for an excellent very tight connection now, but/and: no change, problems still there.

6. Sprayed brake cleaner near all injectors, both plenums, and at the end of all rubber hoses as connected to the intake. The idle speed did not changed that way a single bit, indicating to me that there are at least no vacuum leaks on those places.

7. With a stone cold engine the next day, I did a compression test. For each and every cylinder I cranked the engine for eight (8) audioable cycles and then took the reading:
Cylinder 5: 12,1 bar = 175 PSI
Cylinder 6: 11,8 bar = 171 PSI
Cylinder 7: 11,8 bar = 171 PSI
Cylinder 8: 12,3 bar = 178 PSI

​​​​​​​ Cylinder 4: 11,0 bar = 160 PSI
​​​​​​​ Cylinder 3: 10,8 bar = 157 PSI
​​​​​​​ Cylinder 2: 10,8 bar = 157 PSI
​​​​​​​ Cylinder 1: 11,3 bar = 164 PSI

Re-testing cylinder 5 (just so to make sure that during all this cranking the starter motor etc. did not got weaker - and it did not): 12,1 bar = 175 PSI.
- No idea why the whole of the passenger side has weaker readings than the driver side.
- When inspecting cylinders 2 +1 with a borescope, I could clearly see carbon build up on the whole top of the piston everywhere - but that is as expected on these GTS engines as I have understood from all readings and reports here (not having the pistons oil holes drilled by the factory on the GTS engines definitely does not help with this...).
- None of the cylinders dropped their reading when observed for 30 seconds, and when cranked more after that: each cylinder had then a higher reading of typically about 2+ bars more (i.e. then they would all read in the 190 - 200+ PSI range).

8. Then I bought brand new NGK BPR6ES spark plugs, and put those in - as per Malcolm's suggestion:
​​​​​​​

The initial experience was very good: I good rev as quickly as I wanted and no more stutters/buckling/mis-firings, yes yes yes! But/and: after test driving for about 7 km's (at that time the car was fully up to temperature and oil pressure was a perfectly normal 2.5 bar @idle and 5 bar when giving throttle) and having revved full throttle perhaps 5 times: all symptoms came back in an instance with a rich reading on my WBO2 TechEdge of 11-12 AFR when such happens

Took the spark plugs out, and this is how they looked, remember: only 7 km's from brand new as per earlier picture:



9. Swapped out the EZK with another working EZK: no change, problems still there after having driven another 8 km's or so. I noticed that whenever I revved over 3600 RPM and the flappy opened, the engine really did not liked that and always immediately went into mis-firing/stutters/buckles operation.

10. Swapped out the LH with another working LH: no change, the problems showed up at idling (dropping the idle to every now and then to under 500 RPM) so I decided to not even test drive it.

Note: I did looked at the ICM relay all the time, but it never put either the green nor red led's on. So not likely to be an ICM relay or either or both of the ICM temperature probes problem.

Then I decided to measure all the pins on the EZK and LH module connectors once again, and:
- Temp II and temp I water bridge sensors OK.
- All other wiring's and sensors also measuring correctly.
- TPS idle switch is working perfectly.
- TPS WOT switch is NOT working at all! I have spend the last few days trying to isolate what that problem is:
  • Wiring was all good from the TPS connector to the EZK and LH connectors,
  • As part of my intake refresh, I had bench tested the new TPS switch before I put on on the throttle body (worked fully fine), and when it was on the throttle body (worked fully fine, with the TPS WOT switch = on at around 2/3 open throttle plate, exactly at the same spot as the old working TPS).
  • So left over, and this being the problem with it: at current full throttle = pedal to the metal and kickdown switch underneath the pedal also actuated, the TPS WOT switch is still not opened. Only when I pushed the throttle quadrant a little bit further still by hand (like: a few mm's more) is the TPS WOT contact made. --> This does not make much/any sense to me, because it should open much earlier I think - as I also bench tested...
What do I plan to do and test out next:
- Adjust the throttle quadrant cables, as per Greg Brown's excellent post #10 here: https://rennlist.com/forums/928-foru...l#post10498854
- If that still does not help, check upon possible "hidden vacuum leaks" from the black solenoid that opens to vent the charcoal canister, as per Greg Borwn's post #30: https://rennlist.com/forums/928-foru...l#post15198847
- If that still does not help, do the injector tests as suggested by Fred.
- If that still does not help: I am then out of idea's....

I have a gut feeling that my current TPS WOT switch not working as it should, is not the root cause of my problems that started "out of the blue" 1+ year ago. So any other idea's and insights in the meantime I welcome with wide open arms!
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Old 08-10-2018, 03:58 PM
  #54  
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Arnoud,

Four cylinders on one bank with low compression invariably suggests a cam timing error.

My S4 consistently hits 185 to 190 psig on all cylinders and I am suspicious of that- one presumes that when doing the compression tests you open the throttle plate to full open.

The throttle position switch should not cause your issue but if the wide open switch does not work the brains will not go to the full load maps and you will lose a little top end. Failure of that contact is common place.

Whether cam timing explains your base problem remains to be seen- you need a set of the 32VR cam timing tool and PK bumpstick to check the cam timing that or cam covers off and in with the dial gauge- the 32VR kit is much easier!
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Old 08-10-2018, 04:04 PM
  #55  
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Forgot to add the compression test has to be done with a warm motor so that would explain the lowish pressures but not the difference between the banks
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Old 08-10-2018, 04:11 PM
  #56  
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Originally Posted by FredR View Post
Four cylinders on one bank with low compression invariably suggests a cam timing error.
No doubt this explains the lower readings on that bank. That bank of cams are retarded - the intake valve is closing later than the other side.

I don't think this is your stutter problem though.
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Old 08-10-2018, 04:29 PM
  #57  
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Arnoud,

The stock fuel maps at full throttle give an AFR around 12.4. If you are only briefly pushing the engine rather than holding it, the AFR may well drop a bit as the system throws in evenmore fuel during acceleration to prevent a lean out condition occurring.

Regarding your plugs, as I am aware the GTS motors were specified with 3 prong plugs, whether that was a good thing or whether they run better with a conventional single electrode I have no idea. I run the Bosch plugs WD7 or whatever the stock spec is for the S4 motor, I have also tried plugs one and two heat ranges colder but felt for my needs the stock plugs were best. Perhaps Malcolm is advising you what he finds works best for his GTS.
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Old 08-10-2018, 05:43 PM
  #58  
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Nice intake refresh.
I have a similar issue with the running, on this 1988 Auto i am working on.
and after replacing everything except the rebuilt LH it turns out the LH is failed.
My guess is that this part was installed prior to doing an intake refresh in the hopes it would restore the running,
but their were too many other things that were wrong like shorting to ground Hall sensor and knocjk sensor pins.
These things then damaged the new LH.

Before swapping in the LH My car will start easily and hold an idle from cold to hot its smooth,
If I try to give it gas it stutters and pops and will get up to about 3400 RPM it will then usually cough and die.

I swapped in a good LH disonnected the battery and then connected it,
and it ran rough for about 30 seconds then returned to stable idle and it will easily rev to redline and it works as it should.
I would suggest to adjust the cam timing first,
then fix the TPS issues then take your LH to another car and see if it causes the same issues.
Goodluck.
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Old 08-11-2018, 02:29 PM
  #59  
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Arnoud - I'm still thinking oil ingestion. Experimentally you could remove the two connections to the throttle body - so from the pass cam cover and from the oil filler top - rig those to a catch can instead & and plug the feed lines to the throttle body. See what happens then? I think you can do this without major disassembly.

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Old 08-11-2018, 02:59 PM
  #60  
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Arnoud, check cam timing and put Ngk BUR6ET plugs,,triple electrode like porsche recommend for GTS.version.cold engine readings are pretty good Put new tps and check connection in the CEB. I think that tps signal goes from F or G electrical plug, and can give bad and Intermittent misfire.
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