Sudden Cold Start Problem

[Dano944]

My ‘83 944 always started like a champ when cold. Suddenly, it starts fine but won’t stay idling unless I keep the gas pedal pressed, and once it runs a bit, it idles just fine. Any ideas?

[944 Ecology]

Look for a vacuum leak. Remove the J boot and inspect it closely.

[Dano944]

Originally Posted by 944 Ecology

Look for a vacuum leak. Remove the J boot and inspect it closely.

I just had my 944 smogged. Maybe that boot was tweaked during the smog test?

[944 Ecology]

Originally Posted by Dano944

I just had my 944 smogged. Maybe that boot was tweaked during the smog test?

Possible.

[walfreyydo]

Vacuum leak, IACV or DME temp sensor.

https://www.clarks-garage.com/shop-manual/ts-01.htm#cold-idle

[Dano944]

Originally Posted by walfreyydo

Vacuum leak, IACV or DME temp sensor.

https://www.clarks-garage.com/shop-manual/ts-01.htm#cold-idle

Thanks guys. I’ll check for obvious vacuum leaks, but the car is due for its first minor service since I drove it home, so I will have the Porsche service shop check for all the technical options that I can’t check. 👍

[Chalt]

Since you say it idles fine when warmed up, check the DME Temp Sensor (NTC II).

It tells the computer (DME) what the temp of the coolant is. The DME then adjusts the fuel delivery accordingly. When the engine is cold (sat overnight), the DME Temp sensor tells the DME that the coolant isn't warm and in turn, the DME richens the fuel mixture until engine is warmed up.

When the car has been ran recently and the engine coolant temp is still warm, the sensor tells the DME that the coolant is warm, and in turn, the DME doesn't enrich the fuel delivery like it would for a cold start.

Now, when this sensor fails, it fails towards making the DME think the coolant is warm when in fact it is not. So with a failed or failing sensor, the DME is not giving enough fuel on a cold start (a lean condition) and you will have to press the accelerator to keep it running. (It's failure mode is for the resistance to drop lower)

This isn't the ONLY thing it could be, but all it takes to check this is an ohm meter/multimeter.

Clark's Garage says the Resistance should be 3.3 k-ohms at 59°F or 1.46 k-ohms at 86°F.

Mine went bad slowly over time, not overnight. The symptom was that it became harder and harder to start it cold. Eventually, I could only start it with starting fluid. When I figured out what was wrong, I tested my sensor and it tested at 17.7 Ohms room temperature (about 70F). My new Bosch sensor I got from Pelican tested at 2.17K Ohms at room temperature. Once I replaced the part, the problem was solved.

Here is the Ohm test on the bad sensor. (Range at 200 Ohms on meter) Reading shows 17.7 Ohms.





Here is the Ohm test on the new sensor. (Range at 20k Ohms on meter) Reading shows 2.17k Ohms (2,170 Ohms).




Shows sensor and part number:




Shows Connector:




Location of Sensor: (This is not my engine/picture, it is Ish_944's from Rennlist)

[Monkey Wrench]

Chalt, Thank you for that. This is great thing for us to know and so easy for us all to check. Im going crazy fighting some similar issues with a 88 Volvo and it might be a similar issue, I'll check that sensor ( it's equivalent) out there too. That was helpful.

[Chalt]

Originally Posted by Monkey Wrench

Chalt, Thank you for that. This is great thing for us to know and so easy for us all to check. Im going crazy fighting some similar issues with a 88 Volvo and it might be a similar issue, I'll check that sensor ( it's equivalent) out there too. That was helpful.

You're welcome. When I was having that problem, I was going crazy too, checking fuel pressure, fuel injectors, spark plug wires, distributor cap, the idle control valve, the speed and reference sensors. Everyone kept telling me it sounded like a vacuum leak so I was chasing that too.

I bought a lot of diagnostic tools during that time. A small $30 oscilloscope to check the speed/ref sensors (they were good), spark testers, fuel pressure hose and gauge, a borescope camera to look inside the cylinders, a leak-down tester. I don't regret buying any of those tools. I learned a lot about diagnostics on these old german cars.

The bad thing is that many DIY mechanics just start replacing everything hoping they will fix it. A lot of money is wasted. So many posts I read on a no-start is "I replaced the DME Relay, plugs, wires, distributor cap, ignition coil, fuel pump, fuel regulator..." the list goes on.

I was determined not to do that; throw parts at the problem. On the speed and reference sensors, the o-scope was so worth the $30 because if you just say aww, I'll just replace those two sensors because they're old, a lot of people found out they are hard to get to and sometimes they WON'T come out easily. Some people broke the bracket they go in. Others end up buying or DIY making a vise-grip slide hammer to get them out because when they get very stuck that is what it takes to get them loose.

This youtube video is short and to the point. It talks about symptoms of a failing ECT/DME temp sensor. The video starts at the part that is relevant: https://youtu.be/PYmRuc3ZVdw?t=119

Try a youtube search "volvo ect sensor" --- I saw a few videos there for Volvos.

[Dano944]

Thank you for this info.The temp sensor is cheap. I’ll buy one to have when I take the car in for service. And come to think of it, the cold-start problem was progressive, not overnight.

[Monkey Wrench]

it's a bit unrelated but over the weekend I found the startup and low power issue on my volvo, was that one of the bosche injectors was stuck open. I was swapping all sorts of used parts in and going nuts looking for things like an air leak.. i had checked the plugs and they looked ok two weeks ago , but i suspected it because that plug, although not fouled , smelled of gas and I also saw that the exhaust, while it's normal to see some steam or condensation on startup I had more white smoke than normal.

I pulled the plugs again, and this time # 3 was black like soot! i thought I had swapped the whole fuel rail with other injectors, , but might have actually mistakenly put the same fuel and injectors rail back on, or the one I installed had a stuck one too. ;-)

I pulled the fuel rail off the engine , put down paper to catch it and cranked the engine (coil unplugged)

what I saw was # 3 sprayed a stream of gas, like a squirt gun, It wasn't even being atomized, and when shut off it continued until it had emptied all the pressure out of the fuel rail. It was basically akin to unscrewing the spark plug and dumping a teacup full of gas into the cylinder every time I turned it off. and also running a steady stream of gas with no pulsations. It was basically hosing down the cylinder with gas as it was running. Probably mainly running on 3 but also fighting the compression with some intermittent firing, I think it actually started firing in there once it cleared out, but way way too rich..
I think the issue had been there but gotten worse over time. It sort of crept up on me.

many versions of service notes don't suggest doing this test because of fire risk and it's the first time I've tried doing it like that. Once I did my problems became very obvious.

Now with a different used injector it runs normal. I'd check that again before dumping too much into parts. Fine to replace some that tend to go after 35 years for reliability. good to get the O rings for the injectors and have those onhand. They are cheap. I didn't even concern myself with the spray pattern since I had limited time outside before dark. I could continue on to look more closely at the spray pattern and there are parts to refurbish the injectors or rebuilt ones with a core charge.

the Porsche has a bit different style injectors but I think it could suffer the exact same issue. If you see a cloud of white smoke for the first block after startup, check for a stuck injector. I'm happy now because all is fine, it runs ok now, and I've been trying to figure out why it ran so poorly for quite some time. That test only takes 5 mins if you just get new O rings first. the old ones will be hardened up.. wet them with oil, that saves a bit of struggle inserting the fuel rail.

A "real" mechanic might find it by doing the leak down test with a gauge, but not everyone has a fuel pressure gauge in their back pocket, that's set up to connect to the metric fuel line fittings. I think if there is a little dribble from one that might be harder to find as the leakdown would take more time.

with being able to actually see the ends of the injectors visibly you can see if it dribbles gas after the key is off with the rail still under pressure.

As the issue worsened for me, It became more obvious. I bet there are a lot of cars with less obvious versions of the same issue. I could probably do this test again and take a bit more time and look for any dribble after the key was off, but the problem here for me, was so bad it was just an obvious and serious issue to the point where it ran terribly.

Don't start a gasoline fire, have an adequate extinguisher at the ready. There is an obvious fire risk , so think that through and be safe.