89 Carerra Hot Start Problem

[JPJIII]

What an interesting weekend! PTL it runs. Unfortunately I didn't have a digital camera to record the bits and pieces so all could see the sad state of the sensors I replaced. When I began removing the sensors the insulation had small cracks in a few places and I thought..this must be the issue. However, when I got into it in earnest the insulation fell off in chunks. Apparently 17 years is a bit more than Porsche designed the wires to last. My guess is the cracks allowed dirt and moisture into the wiring and on occassion shorted the thing out. With only 2.0 volts to work with that would be very easy to happen.

Now, lessons learned in replacing the two sensors:
Plan on at least one sensor being correded into the bore of the fixture which requires removal of that piece.
Plan on removing the shock so you can get to the fixture for the remove/replace,
If you happen to have a bore brush for a .410 or 16 gague shotgun it'll be very useful for cleaning the dirt/corrision out of the bores; a little gun grease in the bores makes it ever so much easier to install as well,
Plan way ahead to have a suitable .8mm/.030 washer or metal stock to epoxy to one of the old sensors to set the gap for the new sensors- this is mentioned in the Bently but if you don't have quick set epoxy and the sheet stock on hand it'll take a while to assemble those pieces,
I used a straightened coat hanger to feed up into the engine bay and attached a cord to that to pull though and then tied the cable end to the cord to pull up into the bay. You can reach back behind the engine to feel where the cables come through but cannot see that port at all.
Be very careful in pulling the cabels across the top of the engine to make sure they stay under the accelerator spring and if you discover one is on top of that spring after its all buttoned up take the cables apart to fix don't take the spring off to reposition. That approach will add about an hour to the process once you loose grip on the spring and have to find, then fish it out and then put it back into place. It really did seem like a good idea at the time.
If you can make the cable stay on the intake manifold function to hold the sensor cables and CHT cable you get a gold star from me! I gave up on that and used tie wraps to secure the cable stay around them and then another to secure that to the fuel rail as by that point my get up and go had gotten up and left.
Silicon spray lube works really well to lube the grommet and allow it to slide back into place securing the cabels in the port into the engine bay.
It occurred to me in the disassembly stage that it would be exceedingly bad form to drop anything into the bell housing as the cap head screws that hold the sensors down or the 10mm bolts and washers that secure the base to the engine would probably cause extensive damage to the clutch/pressure plate/fly wheel and I really, really didn't want to pull the tranny off to get a crummy little bolt out. I taped sockets to drives to make sure they didn't come off and took the bits off by hand as soon as they were loose enough. In retrospect stuffing shop rags into the space would be a good precaution as well.

Once it was all reassembled and back on the ground I was very thankful to ever kind Providence that it started and, subjectively, it seems to start more quickly and to run more crisply as well. The engine seems to have a raspier note and I might have picked up a couple hundred revs as well as 60 MPH seems to take a bit less engine than last week.

Summary: At this point I don't know if the basic problem is solved or not. Only time will tell given the intermittant nature of the issue. I do know the sensor insulation was shot and if it wasn't the sole cause it must've been contributory. Also, I've got at least 10 more years on those parts before having to do it again:>)

For those with similar problems with starting take a long hard look at the sensor wires but don't!!! touch them unless you've got replacements in hand. If there are cracks the stuff will just flake off in gobs if you move them at all.

John

[wcc]

It's been about a month now. Has your hot starting issue gone away?

[JPJIII]

I've finally come up with a cause for the no start issue but first additional repairs. Last Fall the throwout bearing elected to disassemble itself while I was sitting in traffic. I initially thought the rubber center clutch had given out but it turned out to be the T/O bearing coming apart. I now have a new spring centered clutch, T/O bearing input collar on transaxel, upgraded shifter fork thingy, new seals on the oil cooler, temp sensor, breather cover, cleaned and repacked CV joints and new covers on the inboard side and new starter. That was an interesting project!

Now for the intermittant no start. I mentioned the issue to the real Porsche wrench that saves me from myself and he suggested the fuel pump might be on the way out. Replacing the fuel pump didn't solve the problem, but I've a new fuel pump now. This after noon it wouldn't start. As usual it spun over just fine but would not run. At the suggestion of a co-worker I opend the gas cap and allowed the tank to vent, hit the starter and it kicked right off and ran fine. Obviously there's a vent problem and the gas tank's not getting enough air into it to avoid building a vacuum. When the vacuum is strong enough the pump can't pull against it and there's no fuel for the engine. It also explains why it 'self cures' the blockage is partial and over a period of time allows enough air into the tank to relieve the vacuum so the pump can pull the gas out and send it on to the engine. Anybody got suggestions as to a most obvious culprit to bollix up the vent system?

jpj

[ischmitz]

wew, it only took 8 month to respond to Bill's question

[JPJIII]

Yep, of course its the first time its crapped out on me since then too :>) The way things work, now that I've got a solution, it'll probably never happen again, unless its a thunderstorm, sleet, hail, snow, stuff like that.

[Carl_Boehme]

Typical hot start problems are related to devices that allow the control pressure to bleed off when the fuel pump isn't running. Check valve is one, pressure accumulator another.

During cold start the cold start injector injects some fuel into the intake (kind of like pumping the gas petal on a carburated engine). for hot starts the control pressure should remain around 30-40 psi for about 30 minutes after the fuel pump stops operating. If it doesn't the engine won't get fuel when you try to start it, because temperature sensor keeps the cold start injector out of the system as it should..

During normal driving the fuel tank is under slight pressure. This comes from the FI system returning fuel to the tank under pressure.

[Richard Burns]

When you repaired the clutch you may have put the earth strap near the gearbox back properly giving a proper grounding for the electrics. I would check that it has not come loose as it can effect the operation of your ECU.

[Richard Burns]

John,
Momo 3.2 has a similar problem. Look at the reply by TroyGT his last paragraph eludes to the electrical grounding problem. This may help you as I had a problem like this in another car and a faulty earth strap was the easy fix.
Good luck.