Bill Ball

OK, heat could be causing seals to leak, or parts to expand and allow air in.

There are many ways to test the intake for air tightness. Your mechanic may have specific tools, such as a smoke machine, etc. However, look at the first part of Dwayne's monumental intake refresh bible. He shows how he did it.
http://www.dwaynesgarage.norcal928.o...%20Refresh.htm

Dwayne's intake refresh guide is like reading War and Peace. If you do half the stuff he did, you'll be doing more than anyone else except Dwayne and maybe MrMerlin, our leading WYAIT guy.

Personally, I don't buy that your problem is intake leak-related yet. You have eliminated the MAF and a few other components with diagnostics, so maybe it has to be the intake All you have said, I believe, is that you have some loss of acceleration once the engine gets warm. Have you observed anything else at all related to this that would help us help you?

RKD in OKC

After owning several different Porsche cars and taking them to VERY good mechanics, I've earned the reputation for being the guy that notices something wearing or going out WAY before it needs any attention at all. I've taken a 5 speed to them saying that the shifter feels like the gear oil in the transmission is getting old. It was and they just laughed at me and said I was the only customer they had that could feel when it was time to change fluids. It was several K before the scheduled maintenance.

OJ GTS

Bloody hell, that is a bit of a read, but extremely helpful and I will definitely use it, thank you!

Like you say I was leaning towards the intake (particularly the IACV) originally for the exact reason that I had pretty much eliminated everything else, but I couldn't think of a reason why it would cause such an issue? In fact although I'm still going to get the intake refreshed at some point anyway, if the car were running lean because of leaks I'd expect it to run worse in cold weather with more dense air!

Really, apart from this the car is 100% well behaved, it idles well at the correct speed, no hesitation or stumbling, and tip in is very clean now it's got a new MAF. Even a dyno before all the work showed it was pretty much in line with the other Auto GTS's on here.

If I were being extremely picky I'd say there was one occasion about a month or so ago where the idle hung at about 950RPM for about 5 seconds before it settled when I stopped at a set of traffic lights after a good thrash, but the car hadn't been used for a few weeks and it hasn't done it before or since, so I assumed it was just a glitch or something sticking briefly.

Perhaps it's just the foam intake tubes of the GTS, maybe they just absorb the heat from the engine bay and are still releasing it when you're accelerating free of the traffic that caused them to heat up in the first place?

jcorenman

On the intake-refresh question: The two approaches are to identify and fix or replace whatever components need replacing, just like you would do anywhere else on the engine; alternately, pull it off and replace and repaint everything. Neither approach is right or wrong, the only issue is the time it takes to get to the bits that are under the intake-- some hoses, the idle valve, throttle switch, the knock sensors.

My preference is like yours, analyze the problem and fix what needs fixing. That's a curse of my engineering background which usually means that it takes me days to fix what others can put right in a few hours, using the shotgun approach.

But as a general rule, if the paint is all peeling off the intake, then there is a good chance that the car has seen lots of high temperatures and replacing all of the soft bits under the intake is a good preventive measure. If the paint is still silver and not brown, then the hoses are probably still rubber and not charcoal.

Now, back to the symptoms: If the car idles fine, and is soft on power only at the top end, then that is NOT a problem with the idle valve (ISV) or an intake leak-- those things cause problems idling. Once you put your foot in it and the throttle opens a 85mm hole in the side of the intake, even the biggest air leak no longer amounts to much.

It's also not the intake tubes, same reason: Once you put your foot in it, the air is moving very fast and the temperature isn't going to change much. Again, that would be an issue with idle, not WOT.

I am also not buying the running-lean theory, but it is possible. The reason I am skeptical is that the factory tune is notoriously rich at the top end, like 12:1 AFR above 4000 RPM, WOT-- it would take a lot to get that to be lean. That's assuming the WOT switch is working...

Which is certainly worth a look: If the WOT switch is intermittent, and not making contact when the intake is heat-soaked, then two things happen: The LH doesn't make use of its WOT map, which normally adds additional fuel to the mix at WOT, so it could be running lean; and the EZK doesn't move to the WOT ignition-map, which I think means it will be missing a bit of timing-- I'll have to check that, for a GTS.

You can check the WOT switch with a voltmeter, but if it only mis-behaves when heat-soaked then it is going to be hard to catch in the act. You can pull the back off the LH or EZU connector, get a meter probe on the WOT wire, and drive around until it misbehaves while watching the voltmeter and trying to avoid a wreck. Or you can hook up a SharkTuner, that will show you all of the sensor readings in real-time, but again you will need to catch it in the act. (The SharkTuner can log the data, so at least you can avoid the wreck).

Or, you could just replace the WOT switch, and while you are in there... Which I think brings us back to where we started ...

Good hunting!

Cheers, Jim

dr bob

Yesterday morning I started then erased a post to this thread, admonishing some the group for the rude welcome. I killed the post because it would only serve to stir things with no obvious instant good result available. I'm not the cop on the beat here, but do disagree some with the way the thread rolled over the OP. 'Nuf said, I hope.

----

I know that there are a lot of doomsday (read: needs intake refresh) posts here, but we so far have ignored the obvious thought that a hot engine breathing hot air is less responsive that a warm engine breathing cool air. The GTS is blessed with a rather heavy (thermally) set of intake tubes, and of course the metal intake manifold. At low engine speeds and therefore low airflow, these tubes and the manifold are warmed by heat from the heads and exhaust conveniently placed immediately below said tubes. The only place where the heat dissipates is to the air flowing through. It takes a few seconds or more for cool air to find its way into the engine. Those few or more seconds are the critical instant response seconds, the ones that define the apparent horsepower of the car, the time when we look for the massive low-RPM torque that separates the 928 from its lesser siblings. Drive the car harder for a bit, get the intake and those tubes cooled off, and your instant throttle response does improve. At keast until things heat up again.

The instant correction for air density is about 15% for intake air temp of 100ºF vs a standard ISO test condition of 60ºF. Raise that to 150ºF heat-soaked degrees, and you've lost about 28%. So it will take 15-30% more actual volumetric flow at the throttle plate to get the same engine response as at 60ºF.

But that's only the beginning. The hot air charge means that there's a greater tendency for low-speed detonation, so knock sensors start to back out ignition timing. The coolant temp sensor has already told the EZK box to go easy on advance. The driver still looks for more performance, so the pedal might go down even further, maybe far enough to get into the open-loop section of the fuel map in the LH box. That decision to switch maps is based on a switch on the throttle plate, not on actual airflow. The open-loop map adds extra fuel to the incoming air, cooling it some and reducing the tendency to knock. But also making it a bit rich, further soggying (new word...) the apparent throttle response.

Conclusion: The OP's observations match what the physics and our knowledge of the engine management tell us will happen.

AO

My ankle feels better today...

I'm not sure I buy dr bob analysis. I've never heard another GTS owner complain about poor throttle response when warm. I think there's more to the story that needs to be investigated.

The OP says the "[lack] of throttle response" is intermittent.

Question: Is the intermittent nature abrupt? Or gradual?

Question: Does this happen only at WOT (wide open throttle) or also when you're gradually rolling on?

Question: What octane level are you using?

Question: Does this happen only in certain gears, or is it across the rage?

Question: Does the check engine light illuminate when you first start the car? Does it ever come on while driving?

OJ GTS

Thanks for a thoroughly informative post! Glad to hear I'm not the only one who suffers from Engineer syndrome!

The paint on the intake looks the right colour and generally any hoses I've taken off in the past have been in good condition, without much cracking, even where they've been clamped by jubilee clips.

I think the WOT switch check is a good suggestion, I would never have thought of that. Perhaps John will rent me a shark tuner for a couple of hours so I can have a play. Again, it's one more thing to rule out! I'll let you know how I get on.

Out of interest, what other parameters does the shark tuner show? For instance will it show me if it's pulling timing? Might give a useful indication of what's going on!

OJ GTS

More than likely!

If I had to have a guess then I'd say it felt like it was pulling timing, which would presumably be in line with knock sensors triggering.

OJ GTS

Glad to hear your ankle is better!

It's definitely not abrupt, it doesn't suddenly feel like the power has dropped off, it just either picks up and pulls cleanly or it feels flat. The speed at which the throttle is applied doesn't seem to have much difference, and it's 'consistent' in that it wont change from pull to pull. As far as I can make out it seems to do it after the car has been driven around at low speed for a while, hence the assumption of heat soak.

I use almost exclusively Shell V-power, which is 99 RON and about as good as it gets here

Seems to happen in any gear, but more noticeable in higher gears, presumably simply because of the gearing itself.

I've never seen an engine management light come on in it, even before it was my car!

Thanks for the help

Vilhuer

'96 was probably last year any 928 could be registered in EU countries as new car. Last remaining GTS had to be registered by the December 31st at latest before requlations changed for '97 MY. It can even be late '94 MY car as last cars sold really slowly. In any case VIN number tells how late car it actually is and gives some indication when it was actually made.

AO

That helps. The next time you start the car, look at the instruments in the upper right corner and see if you see the "Check Engine" come on. It should - along with every other light on the dash.

However, I'm starting to agree with drbob's assessment, though I've never heard anyone else experience this.

Bill Ball

So, the problem appears to be rather intermittent, if I'm reading correctly. You have dyno runs which seem to be spot on. The engine is usually quite warm then. This is going to be hard to find if intermittent.

Anyway, just for completeness, the WOT switch usually works or doesn't (not intermittent) and can be tested with the Sharktuner, Porsche Hammer tool, John Speake's Hammer clone called the Spanner or Theo Jennisken's 928 Diagnostic program OR you can check it directly by getting your Ohm meter out and checking for a signal when the pedal is depressed. You would pull the plug off the LH, hook the meter up to the plug connectors 3 and 17 and look for continuity (well, spec is <10 Ohms) somewhere above 2/3rds throttle. Do the same with the EZK plug at pins 26 and 18. Many of these switches don't trip until the pedal is close to or on the floor. Some don't trip at all which may be a sign the throttle cable has too much slack or the WOT switch is broken. The WOT switch enrichens the fuel mixture and retards timing a bit when you push hard on the gas pedal. Even if you find it broken, which I would definitely fix, it may not be responsible for your complaint.

The Sharktuner certainly would provide a lot of useful information if you could log all the parameters it monitors during one of your low power episodes. However, once you find and fix the problem it wouldn't be much use to you unless you were planning some serious engine mods.

Randy V

Hey, I wished you good luck and imparted some well-regarded wisdom.

If that's rude to you, well, carry on sir.

AO

You'll have to forgive Randy, his car sprained its ankle yesterday.

OJ GTS

I'll have to pull out the log book and check out the VIN when I'm home, I'd love to have a bit of history on it!