Arnoud

The problem:
After the car having worked flawlessly also this whole year thus far, it now suddenly has presented the following two main problems:
1. 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 know these values, because I have a totally seperate WBO2 controller which also measure the engine RPM (via a TechEdge controller).
2. When giving quickly full gas from idle to around 2000 RPM, it stutters/buckles completely - including misfire/loud pops. If instead slowly reving the engine from idle to around 2000 RPM: all works fine.

I think the main clue is in the fact that the idle drops, instead of the many threads that I have found thus far where it goes up.

What else have I measured and/or checked thus far, using Theo's diagnostic tool:
- No single stored fault codes on the LH and EZK controllers (nor any of the other controllers).
- Idle Air Control Valve makes audiable click when tested, i.e appears to work OK.
- Resonance flap makes audiable click when tested, i.e appears to work OK.
- Fuel injectors makes audiable clicks when tested, i.e appears to work OK.
- Adaptation: runs and finishes OK, but does not make any difference to these two problems.
- NBO2 sensor appears to works fine, and shows initial warming up and working mode values i.e. goes from rich to lean when lifting the gas pedal @2000 rpm (and it was changed for a new one during the year 2014, purely as preventive maintenance then).
- Throttle full switch (WOTS): is currently not working, as in: showing always open. I have had this problem before, and that I solved then by adjusting the cable on the trottle kwadrant - but not done so yet this time. However: I do not think that this is causing the current problem at all.
- Throttle idle switch: is working fine, as it always has.
- Temp II sensor: is working fine, and reporting correct values for the LH and EZK controllers.
- MAF voltages: with running engine appears to be OK. Few values as measured: 720 RPM gives 0.39V , 1218 RPM gives 0.82V and 2021 RPM gives 1.39V
- Note: both the MAF and LH very redone by John Speake, several years ago.

What will I check next:
- Both distribution caps + rotors + coils + wires. All those were changed during the year 2014, purely as preventive maintenance then.
- Fuel pressure damper + regulars, I will check for possible fuel smell.
- If needed, pull the intake - as it has never been refreshed thus far.

Anbody with additional pointers and/or hints, what is the most likely root cause for this problem?

Jim M.

Arnoud, I had a very similiar problem on my GTS. The big difference was my idle was stable. Under slow acceleration everything was fine. But under hard acceleration it bucked and stumbled and would not accelerate at all, almost like I had hit the rpm limiter, but no where near the limit. The final fix was replacing the hose under the intake. The hose had been replaced with a new one at a recent intake refresh but The replacement hose was about 10mm shorter than the original. Under hard acceleration the pressure on the hose escaped to atmosphere. You may be able to see the hose with a borescope. Sean Ratts found the problem and Roger somehow sourced a correct length hose and all was good. Sean may be able to add more detail on this.
Hope this helps

Randy V

When's the last time you gave her an Italian Tuneup?

Alan

Arnoud,
How certain are you that the idle switch is still good? How about the ISV - possible sticking? either could explain the idle but not really the stumbling under gas.

Above 2K RPM is all good? static fuel limitations would impact high RMP more. With quick acceleration you still have high intake vacuum - with slower acceleration - vacuum drops off - FPR/Dampers issues? if you have lots of oil ingestion you can get a slug of oil mist in the intake as soon as the throttle opens (knock count?).

Just wild guesses...

Alan

Arnoud

Jim, Randy, Alan: thank you for your immediate replies and suggestions, and this is what always has and continues to make this 928 Rennlist group so invaluable!

My replies underneath, and I will further follow up in the days to come with addinal tests so to get to the root cause of this...

Jim: with the hose under the intake: do you mean the rubber house from the MAF to the plenum, or another one?

Randy: I always drive it like I stole it!

Alan:
- Idle switch still good?
I am not 100.0% certain that the idle switch is still good, as I have never removed it from the intake thus far. I only base this on the fact that when performing the test with Theo's diagnostic tool, that I can hear it clicking once per second - as to be expected from that test.
- Above 2K RPM is all good?
I did not really tested that, will do so today and report back (my expectation/prediction is, that it will be have the same when quickly reving from 2K to 4K RPM)
- Knock count?
I did perform a short knock count test: @2K RPM while standing still (i.e. no driving load on the engine) it registered 6 knocks for 3966 firings, then I stopped that test. Seems to read OKis (as per Theo's manual stating that 5-20 for 10000 firings is OK for a perfectly good working enigne).

FredR

If I am looking for off idle hesitation the first thing I tend to suspect is something that can cause the mixture to lean out. The obvious candidate is an air leak allowing unmetered air into the system but this is usually accompanied by an idle that is wanting to race and the ISV thus closing to try and compensate for speed and thus a considerably lean mix. If you have a WBO2 sensor [and ST2?] take a look at the AFR to see if it spikes above 15 when you open the throttle -do you have a logging capability rather than just an analogue gauge as the latter may not show what is truly going on. That the rpms are dropping may suggest the ISV is having to work overtime to compensate for a false air in-rush and thus over compensates [temporarily at least].

Also, when you open the throttle the LH initially throws in more fuel to help compensate for the leaning out that occurs due to the air flow acceleration but either way there is a bit of a lead lag syndrome no matter what- I cannot visualise any way this feature could be compromised but if it was there would be a hesitation on pick up.

The full throttle switch should not contribute to your probtlem if it has failed but if it has, you will not make full power when flooring it.

Without removing the inlet manifold, cracked pipes around the ISV are usually difficult to detect but rather obvious when opened up but no one wants to take the inlet manifold off only to find everything is OK..

You state the inlet manifold has not been removed - if that means not removed since the car was new then all I can say is that it sounds as though it is well past the time that it was removed irrespective of whether a cracked ISV pipe is the cuplrit. That being the case it is highly likely the ISV pipes are shot, the breather pipes will be gooey rubber and your knock sensors close to falling to pieces.

Rgds

Fred

bogdann

Watching this as I have the same issue with an 86. Starts and idles well but stumbles at 1800 - 2000 rpm but smooths out over about 2500. Removed intake, repaired all wiring, replaced all hoses I didn't replace 4 years ago which were in good nick as I haven't driven it too much.
Subscribed

Tom in Austin

I predict you'll find an air leak, probably one of the hoses under the manifold. The ISV tries it's best to adjust for the excess air and that produces the idle cycling you're experiencing

dr bob

Predict air leak as most likely, followed by MAF out of range due to age/wear.

On my car, I experienced identical symptoms. It was solved by carefully cleaning and reseating the harness connection to the MAF sensor. The MAF on mine is due for rebuild by both age and miles. The pins for that connector, plus the shell and the boot, are all available as individual replacement parts. Use some DeOxit on the connections after verifying everything is intack there.

SeanR

What Jim M said above and others have also stated. Betting you have a nice air leak, I've encountered it many times.

Arnoud

Fred: thank you for your contribution for my problem.
Indeed: the intake has never been removed from new (as I have all maintenance records ever done on my car) - and was/is on my list to get done as this year upcoming winter maintenance. So I might have to "fast forward" this, once I have eliminated all other possibilities...

Norm: I hope this thread will help you quickly narrowing down your root cause.

Tom: thanks for your provided help as well! It still does not logically explain to me why the idle stutter is to the downwards range "only" once per every ~10 seconds or so - for a rather large downrange of -25%.

Dr. Bob: thank you for sharing your experience, appreciated!
Will check the MAF harness connection in the day(s) to come, and report back here.

Arnoud

Sean: thank you for your advice, which helps me a lot to narrow it down as quickly as possible!

I just eliminated the possibility of it being perhaps distributors or coils or ignition wires related (as Bill Ball had reported in one of his threads for a similar problem, some years ago). All of these are still looking as good as new, as when I put them onto the car during the year 2014:

Drivers side distributor connectors (and yes: the connector to the coil wire looks the same too, although not shown here):


Drivers side coil connections (and yes: the external earth wire is also perfect, although not shown here):


Passengers side distributor connectors (and yes: the connector to the coil wire looks the same too, although not shown here) + one ignition wire (Beru) look (and they all look as good as this one):


The next thing I will eliminate is both power stages on the front of the car, which feed the coils - by using deoxit on the connectors and re-cleaning the ground point there too.
Very unlikely, but a possibility for my problem - see this link: http://jenniskens.livedsl.nl/Technic...6/MyTip669.htm

Arnoud

Ignition power stages connectors + it's ground point re-cleaned, DeOxit applied and put back together (and yes: the two metal springs go on the other way around, as shown resting in the picture here):



but the problem still remains exactly the same. Well, actually: I was expecting this result, just eliminating unlikely - but possible - problems.

Few other observations / checks that I did today:
- Temp I and II sensors are indeed working all fine = providing the right temperature resistance ranges, from both "stone cold" engine to fully warmed up.
- The narrow Band O2 sensor is also working fine: when disconnected with warmed up engine, the LH is fulelling it on the rich side as expected - with my totally seperate Wide Band O2 circuit showing an AFR between 11.5 - 12.8 (between idle 675 and 2K RPM). When going off the gas pedal @2K RPM it shows lean as expected (as fuel is then cutoff towards the injectors) - with an AFR towards 19.
When reconnecting the narrow Band O2 sensor, AFR shows around 14.7 @idle - as expected.
- I have not smelled fuel when disconnecting the vacuum lines from the fuel pressure damper + regulators.
- The total stuttering/buckles is "only" from idle to 2K RPM, meaning: when I rev it full throttle from 2K - 3K RPM or from 3K - 4K RPM (obviously only with warmed up engine) then it does NOT stutter/buckles.

Any bets which hose(s) under my intake most likely will show to have a crack/leak?

FredR

Arnoud,

Have you had a look at the condition of the wiring harness just before and entering the MAF [i.e. under the boot]? You need to eliminate this as a problem just as you have other possibilities.

Once the items outside the inlet manifold have been eliminated it will then confirm what some of us suspect to be causing the issue- my money is currently on a split in the pipes around the ISV simply because the problem is around idle to 2k. lf original fitment they most definitely need replacing as will the knock sensors and the CPS/Hall trigger even if they are working at the moment.

You said the ignition leads, distributor caps and rotor arms have been replaced but have the fuel pressure controller and dampers been replaced at some stage previously? Whereas the smell of fuel in the vac connection is a performance indicator for failure, if those items are still the originally supplied ones, one wonders whether they can possibly be working as intended.

Rgds

Fred

SeanR

Double check the rotors internally if you haven't yet. Had this on an '88. While the car still ran fine it had a stumble. That is ran as well as it did is amazing.

Main coil node is missing.





Pano's had one like this.