Rob in Oz

I've spent 3 days on this so far, searched the posts, RTFM, tested by the manual, checked the connector plugs and checked the voltages. The problem started on Wednesday at a track day (driver training), when at the end of the day, my car spluttered to a halt at the side of the track. Aha - low fuel + fuel surge=stopped engine. Sure enough, after a couple of minutes, the engine restarted and I drove back to the pits. Packed up my gear and drove to the nearest service station and filled up - only took 65 Litres, but fuel surge still could have accounted for the problem. Driving home along the highway, everything was initially OK, but a niggling intermittent stumble/miss set in. In the next stage of the trip, had to cross a small mountain range, OK up the hill, but started cutting out completely on the way down. Pulled up most of the way down with engine stopped. Looked underneath and cat was glowing red hot! I figured then that the fuel pump was fritzed making everything run lean, so phoned up a tilt-tray truck and delivered it home.

Next day, ordered new fuel pump and filter, arrived 24 hrs later. Fitted both and you guessed it - same result. Car starts, idles perfectly at 800-900 rpm, splutters and stalls as soon as the throttle is opened. Aha - that'll be the TPS then! Checked with the multimeter as per the factory manual, all checks out. How about the reference and rpm sensors? Plug connectors OK, resistance and insulation checks OK. Don't have a CRO just now, so can't check the waveform.

Air flow sensor and intake air temp sensor - as per factory specs.

Checked all the vacuum lines (again!) and all connector plugs (again!).

So for those of you who are still with me - what to do next? The perfect idle would seem to rule out basic injector/fuel issues. It appears that the progression from idle circuit to run program is the problem - but it's not obvious to me. Any ideas appreciated.

TIA

Rob

adrial

Try measuring the AFM output @ the DME to see if you get different results. You can pull off the white plastic cover and get to the pins from the back.

That sounds exactly like running the car with the AFM unplugged.

Someone is going to suggest it, because someone always does...so I will get that out of the way now. How old is the DME relay?

Rob in Oz

Adrial

Yep - did that - 5V at the DME plug, pushed air flap through full load range, nice progression to 4.6V at full load.

Thanks anyway - any other ideas?

Rob

Rob in Oz

And yes, I have a spare DME relay that I swapped in and out with no change in results...

Rob

adrial

Only other thing I can say is that I have measured my AFM to be fine, but still had issues. Nothing of the magnitude that you are talking about though.

I assume you mean 0V at the DME plug with a progression to 4.6V at full load?

You mention a glowing red cat...possible meltdown? clogged cat? Take the cap off the CO test port and see if it runs any differently. You could also remove (and unplug) the O2 sensor. I can't imagine that would cause an instant stall when you give it some throttle though.

Tom M'Guinn

Do you tried a known-good dme box?

Rob in Oz

Nope - they're not exactly thick on the ground around here - but it's something to try down the track...

Rob

RKD in OKC

You said Cat was glowing red hot...is it clogged up?

Rob in Oz

Nope - I think it fuelled up on the way down the mountain and was having its own independent internal combustion show. Fortunately I'd pulled over onto an area with short grass - otherwise it would have been fire extinguisher time... There certainly doesn't seem to be any restriction, judging by the flow and pulses at idle.

Rob

OZ951

G'day Rob,

Check the output of the temp sensor near the coolant neck on the head as well. Compare it to the specs in the manual. It can cause some odd fueling behaviour if it goes **** up. Also check the connector.

Do you have an AFR guage & fuel pressure guage, if so are they reading normal ?

It sounds like your timing & reference sensors are ok if it starts and the problem only arises when you open the throttle.

I'd be looking at the grounds on the block and making sure they are ok. Then checking the DME.

leonc944

change your spark plugs to brand new ones.

Rob in Oz

Hi Wayne

As it happens, I was looking at your site today in my quest for more knowledge. Don't have an AFR gauge, but checked the pump output and it's as per factory specs or better. Will check the temp sensor and grounds also - thanks. Nice to hear from you again...

Regards

Rob

OZ951

Rob,

with the fuel pressure guage I was thinking more about the FPR.. checking its delivery pressure.

Rob in Oz

Wayne,

I swapped in the original FPR with the same result, but I do think I'm dealing with a rich condition off idle.

Rob

Rob in Oz

Still working on this a week later (have to go to work etc...) and after speaking with the service manager (good guy) at the local Porsche dealership, he suggested that the injectors might be just passing sufficient gas at idle, but being restricted as the demand went up. Pulled the injectors and had them tested and resealed. Initial flows were 98-100%, cleaned and tested to 100%. Obviously no problem there! One positive out of this was that on pulling the fuel rail, I found that the damper to fuel rail hose was cracked underneath, so had a new one made up, possibly saving an engine fire down the track...

Talked to another Porsche tech at a meeting last night and after scratching his head, he remembered a 928 that had a shorted O2 sensor, that was heating its cat to red hot. So - checked the O2 sensor output at the connector - doesn't seem to be producing any voltage (should be 0.1 - 1.0V depending on A/F ratio) at idle. However, it makes no difference to the off-idle condition whether or not the sensor is connected - it still stumbles and stops when the throttle is just opened. According to Clark's Garage, the engine should run with the sensor disconnected, thus eliminating (?) this from the stumble-stop condition. Or is this just for N/A's, with turbos having a fail-safe over-rich setup for when the O2 sensor fails?

Oz951, if you're reading this - I checked the output of the temp sensor as you suggested and it's in spec and varies with temp from engine cold to warmed-up. Good thought, though.

Just for kicks, I pulled the (Guru) chip from the DME and replaced it with the original - no effect.

Also pulled the inlet manifold for a look-around underneath and replaced the manifold gaskets - nothing unusual there.

So at this stage it looks like possibly the O2 sensor (looking for input here) or the DME. Just waiting for a friend to return from a break to try his DME...

Rob
1986 951