blueblooddude

Hey Dave,

Great questions, I'll get the answers for you. I believe it was idle only measurement, but I'm not sure what he tapped into to get his readings. Also, I'll have him check to see if there is a test port. Did it surprise you that there is only one sensor?

I can't believe all the issues that I asked to be addressed worked out perfectly and I have a new and increasingly serious problem made for me! Part of the reason why I went with this car after a 1.5 year search is that it was rock solid mechanically so I could baby it with the cosmetic stuff.

ERRRRRrrrr...don't mean to whine, just venting.

Regards,

Jeff

blueblooddude

Hey there Ernest,

Serious as can be dude. Is this not normally done with the 928s? If so, is the tach toast and may be the major contributor to the problem? Have you had a tach register off, and if so what is the action normally taken? Replace it? Like I said, I could have lived with 300/400 RPMs off...

Jeff

ErnestSw

It sounds to me like the mechanic who's working on your car hasn't got a clue. You can't "reset" a tach by moving the needle and the rev limiter is not a device, but a point in a piece of software that causes a reaction. The issue is how the software gets its input.
The 928 has (I'm guessing) about twenty parameters that determine idle speed. Some are electrical, some are mechanical and some are vacuum actuated. The Idle speed and rev limiter are hard wired into the brain of the car at a fixed point IF AND ONLY IF all of the inputs are normal. Troubleshooting an idle problem is a matter of figuring out which inputs are giving the brain the wrong information.
You don't have a tachometer problem, you have a mechanic problem. He either genuinely hasn't got a clue or he smells a completely naive consumer with a VERY deep pocket.

blueblooddude

Hi Ernest,

Thanks for the input...unfortunately, I supsect it confirms some of my worse fears however. I'm not exactly "completely naive" but this is certainly my first Porsche, and my first specialty car in New England...I had a couple of Ferraris when I had a place in Florida, and have had a couple of Lotus over the years. Thus, I have had to tap into a new repair network up here...this guy came independently referred to me by several Porsche collector friends as well as other German car owners so that is whay I am so surprised by this situation. I don't think he intentionally tried to get into my deep pocket as he CREATED this problem and he's not getting anything more from me...but I do sense that he is now out of his league with the electronics, and that concerns me because if it's got to be shipped to another shop THEY are certainly going to try to get into my pockets.

When I was younger, I enjoyed getting under the hood and doing things myself...chasing problems and learning about each engine idiosyncrasies was part of the fun. Unfortunately I don't have as much fun for "projects" right now, thus my selection of this car. I can still have fun with the minor cosmetic stuff, but I wanted a car that will start and run for some fun when I turn the key. Looks like I'm heading back into the deep end of the problem pool...

So...the tach, one of the 20 or so points in the "nervous system" has been tampered with and triggered this problem. What's the next step? Regardless of who does it, do I replace the tach and the system will straighten itself out?? Or is this an electronic/ECU fix now to correct a mechanically induced problem? If so, I do indeed have to bring it to someone else...

Thanks again man, it is certainly frustrating...

Regards,

Jeff

blueblooddude

Dave,

Spoke with the tech. He hooked his computer to the spark wire and tested at idle speed.

Regards,

Jeff

WallyP

First - there is no way that I can think of that moving the needle on the tach would affect the running of the engine.

Second - The 928 uses only one flywheel sensor. The second sensor on some models is used for testing only, and has nothing to do with running.

The standard rev limiter action is pretty noticeable - the engine will cut off and on rapidly, causing a jerking action.

I would suggest that the tech hook up an inductive-pickup timing light, and rev the engine until it cuts out. The presence or absence of spark will be obvious. If you have spark, then you are probably losing fuel. This test will be quick and easy, and will cut the possibilities in half.

dr bob

As much as all have focused on the 'moving the tach needle' issue, my instinct is to go look at the "clean the engine" detail mentioned in the owners initial post. Cleaning the engine has been a mixed bag for me over the many years and many cars. Most of the cars seemed to enjoy the bath process and the owner (usually me...) seemed much happier with a spotless engine bay. But there have been a few litlle exciting sessions.

In one, my Subaru mountain ski car refused to run after the bath. A few hours of detectiving showed that I had spark, had fuel, everything appeared to be fine. In a scattershot fit of brilliance or lucky-guess syndrome, I decided to pull the distributor and put it in the oven overnight at 150 degrees. Let's make sure it really is dried out in there. Next morning, car fired right up. Hmmm...

In another, the car ran really rough after the bath. Cured with new plug wires. Whatever was washed out of the wires was conductive, or the grease and slime were insulators. No matter, under load the engine wouldn't rev.

Another episode involved discovering a cracked distributor cap. See a pattern growing here?



What I Would Do:

Start off by, at minimum, removing and cleaning the distributor cap(s) and rotor(s), inspecting them for any cracks, carbon traces, missing rotor buttons, etc. At the same time, remove at least the plug end of the wires and look down the hole to see how much has accumulated in the well around the plug. Get that cleaned out, remove the plugs and clean the exterior of the insulator, and replace the plug wires using a little dielectric grease on the boot where it slides over the plug.

Figure out if the air filter got wet, and decide if the MAF might have been dameged by water. John Speake is the Master Guru of MAF sensors and may want to touch on what can happen and what to look for.

The temp sensor II on the coolant crossover pipe is sensitive to water. You can verify the function with an ohmmeter and your WSM resistance vs temperature charts. A little water in that baby will keep the car from running well, may or may not be the cause of the poor engine running at speed.


As others have suggested, go back through every detail of what your tech did to the car. Solid troubleshooting starts of with a list of what might have happened since everything was OK, so build that list from your techs "what I did to it" list. Decide if any of those would cause the problem you are seeing. Prioritize your list based on probability of that causing the failure, then on ease of testing that item on the list based on parts price/availabilty and the amount of time to test. The last item you check will be the one that cures the problem of course, so decide whether you want to invert the list, or just stop testing when you find the cure. Tough decision... I try not to invert the list.


Oh, and on that tach thing... The two least important readings on the tach are the no-speed and idle-speed indications. A couple hundred RPM's error at idle is a needle-width. Where you really want it to be accurate is at the mid to top end. Pulling the cluster to "recalibrate" the needle at idle is the same as someone reaching into your wallet and stealing your money. Plus, now you have some ncertainty about how accurate the thing will be. Fortunately, it's only likely to be off by that same several hundred RPM's at speed. Could be worse-- it could be off by that same 25% at speed. Count your blessings, and spend some time counselling your tech.

blueblooddude

Hi Wally!

Hey, I passed your info on to the tech this afternoon and chatted with him at length. He definitely concurs that the problem is fuel loss...

Your wisdom is sound I think! I actually was thinking the same thing myself after stewing a bit this morning. While I'm so irritated, it's hard to believe such a coincidence could occur where the problem developed immediately after the work he did but you are right. Truth is stranger than fiction, and WE'VE ALL SEEN SUCH CASES! He is the first one to admit that he's a good mechanic but past a point is not an electronics expert...his computer equipment only goes so far.

Who knows, maybe the brain developed Alzheimer's and I need a new one or something...I'm going to call and schedule a diagnostic visit with a bigger Porsche shop with more elaborate systems and electronics personnel. You New England guys...I live in Rhode Island and work in Mass, west of Boston. My current plan is to take it down to Inskip in Warwick, RI but if there's a better place, let me know. (I considered buying a car from them but wasn't too impressed with my experience, but they're the best equipped that I know of in the area.)

If it's a brain or other parts needed, I'll be giving you a call Wally!

Thanks again...

Jeff

blueblooddude

Hey Bob!

Great feedback man, thanks! As I mentioned to Wally, I told the tech to put her back together and I'm picking her up tomorrow. Hmmmm...I too have had cars that acted hinky after the engine cleanup during a "burn-off" period but have never had to go to the lengths you mentioned. I'll run through your checklist over the weekend when I have my hands on the car again...it would be awesome if that was the problem and she straightened out and didn't have to make her visit to the electronics guy!

Regards,

Jeff

Jim bailey - 928 International

It would be interesting to simply put a $10 noide light on one of the injector connectors and rev it to 4,000 and see if the signals stop. Same for the fuel pump with a test light. I do seem to recall reading in the manuals that a defective tachometer can cause the LH brain or maybe ignition to act strangely my problem is finding where I saw that having spent far too many hours reading the workshop manuals and tech sheets. Also nice to check the actual RPM to what the tach says at different RPM.

ErnestSw

The level of familiarity with the 928 is far more important than the level of diagnostic equipment available. Ultimately it will boil down to simple reasoning OR be beyond the capability of the garage in the case of the brains or MAF (Mass Air flow Meter).
Someone in the Boston area should be able to recommend a reliable 928 person with the knowlege and common sense necessary to figure it out.

all4woody

Is the engine electronically shutting down or is there fuel loss. I had a problem once with a line that allowed air back into the fuel tank, at high RPM the clogged line would prevent air flow into the tank, and fuel would be limited. This caused the car to sputter when I got on the gas and eventually collapsed my fuel tank.
ps. anyone need a "slightly" used fuel tank for an 86?

WallyP

There have been several times that I have been seriously tempted to get a bunch of 12 vdc LEDs and some speaker wire and build an annunciator panel for the GT. I would have LEDs that show when there is power out of the fuel pump relay, the LH relay, the EZK relay, the ignition switch, etc., etc., etc.

Sure would be nice to just look down at a panel and see what is going on...

worf928

Hi Jeff,
At this point you've read the other posts that describe in summary why moving the tach needle is basically a no-op.

Wally(P), dr bob, Jim, John S., and other folks have chimed in with their usual spot-on advice. And, specifically, dr bob, points back to the engine cleaning as the most likely source of the issue with the tach/rpm thing being - quite possibly - misleading (but annoying.)

You responded with - what I perceive - to be a desire to find someone local (Boston area) to do the work or at least a preference to not invest significant chunks of your own time. In that direction...

I have had some experience with some of the local shops in the last 8 years.

EPE in Natick has a very good reputation. Jerry and William (if he's still there) really know what they are doing. However, I cannot recommend them without reservation for the simple reason that IMO none of them like working on 928s. In my case they tried twice to align my '91 and failed both times. In each case I waited while they did the work, was able to talk with all the techs, watched a PPI on an 88 5-speed 928, and thus formed my opinion that 928 work was not high on their desired work list. They did however, in 1998, perform an intensive electrical troubleshooting task on my '91 with success. (However, that experience and the bill that came with it was the last straw for me for using 'professionals' for anything that required more brains than specialized equipment.)

There are also a few 928 folks that have formed good working relationships with the owner of EPE (Jerry) and are happy.

Autobahn in Peabody does very good work if you tell them precisely what you want. And you want the owner Rick Heathrington (sic?) to do the work or his top electrical guy as they are the ones with 928 experience. They've never work on my 928s. They have done work for me on a VW and Audi in the past.

At this point the only tech that I allow to work on one of my 928s is Bob Delsandro at IRA in Danvers. At this point he does my AC work as I have yet to invest in the right equipment to do that myself. However, he has in the more distant past done other work on my '91 and I was pleased in each case. He has extensive 928 experience and knows what he's doing. However, he will not be cheap. If you are interested in Bob at IRA PM me and I'll give you some further direction on how to get speedily through the normal "Service Manager" procedure.

If I wasn't in the process of finishing-up some extensive work on 928andRC51's '87 5-speed and itching to get some of my own projects done in the next month I'd offer myself for a weekend day for some diagnostics. If, however, a few weeks pass by and your car's problem has not been rectified let me know and if logistics work I might be able to give you a hand.

John Speake

Jeff,
here's a bit of trouble shooting diagnosis you can do for your self, after you been through all the other good advice previously given.

Drive ther car and go to the 4000rpm point where it "cuts out", and try to observe the rev cpunter when it looses all power. It the rpm counter reading drops like a stone, even though the engine rpm is coasting down, then that identifies a possible set of causes.

If the tach tracks the engine rpm down, then the cause could be other things.

Let us know how you get on, and then we can give some more accurate pointers.