no doubt

Does that mean (since the engine was cold) that your cat/engine temp sensor is bad, or that your temp monitoring relay is bad?

waveflux

Logic would dictate the latter. But....i'm guessing.

no doubt

The plot thickens. Was that plug in cylinder (Green) 2, 3, 5, or 8 by any chance?

no doubt

LOL! Logic and guesses are often unrelated.

no doubt

You can first *test* to know for certain...saving an expensive series of throwing parts at the problem.

Here's how:

Underneath your 1989 928 you should have two cat temp heat sensors. They should be identical.

Swap them. (if I'm wrong about the cat temp sensors, then you may have two engine temp sensors, instead...same deal...swap them)

Now attempt to start your car as before, with your buddy watching.

If the Green LED still flickers after the swap, then the problem is likely with the temp monitoring relay.

However, if the problem has switched sides such that the red LED now flickers...then replace that sensor. In that case the problem swapped sides with the swapped sensors. Since the problem followed a sensor, that sensor is probably the problem.

waveflux

Gotcha, I'm all ears tonight for other suggestions. As I will start on this in the morning, would like a list of possible fixes in case I come up empty.

waveflux

IIRC it was # 8

Jim bailey - 928 International

How about you unplug the clear relay and on the plug jumper terminals 2, 4 and 5 that bypasses the relay (leave it unplugged) . See if the car runs OK just be careful because now the cat can get too hot ! If it does run OK then it is either the relay or the temp sensors on the heads. If it runs badly you might have a plug ignition issue.....

no doubt

I'd also suggest acquiring a $2 spray can of starter fluid. Spray into your cold intake a small amount (e.g. 2 seconds of spraying) to help determine if your current cold start problem is fuel or spark related (e.g. does the engine die after the starter fluid has been burned off).

waveflux

Ok..these are very good tips. I should get right to the problem with these tests.

no doubt

Hey Jim, could you elaborate on the temp sensors a little bit? Would the heads get hotter or colder if a spark plug wasn't firing...and would the cat on that side do the opposite?

Jim bailey - 928 International

the sensors are near the exhaust ports on the head in 89 #4 and #8 . The 1990 > they moved forward to # 3 and #7 . The move was because of too many false readings. A cylinder not firing would be cool and too large a difference in the temps triggers the relay. The relay is supposed to ONLY shut down 4 injectors BUT anything with the ability to shut off all 8 can and will do so if it fails just right ! I believe there is a time delay in the relay to give the engine a moment to warm up before it compares exhaust port head temps .... The sensors are seen from under the engine but way up high on the lower head right next to the exhaust manifold. All 8 cylinders have threaded holes (six are plugged) so the 89 car can be updated to move the sensors to #7 ,#3 which is recommended in a Porsche Tech bulletin.

Mrmerlin

Did you ever remove the coil wires and check for seating and corrosion, even though they might be new I have seen more than a few times a coil wire isnt connected fully into the coil or the dist cap




SEE POST #76

waveflux

No, but I'll put it on the list.

no doubt

Thank you. So no temp sensors physically on the cats...