jvandyke

We're getting this weird hesitation when cold, almost every time, it fires up idles fine but when starting out it will hesitate, lose all power and just idle, after struggle bussing for 30 seconds or so it clears and all is well, off we go. Research so far has revealed some possibilities: vacuum leak, FPR,
Strange that it's only when cold and almost completely repeatable.

'87 924s, all stock, all refreshed except not running a fuel pressure dampener at the moment. Fuel system was cleaned and pump newer, filters new but that's not to say something isn't still amiss there. It didn't do this last summer but did when snuck out of storage for 2 days a month ago (break in the weather) and again today (another break in the weather)

I'm remember just now that the O2 sensor wires may have been disturbed as we did the clutch this winter, will need to check that, if not in the loop would the DME do something weird when stone cold for a minute until it ran it open loop?

V2Rocket

Check cold fuel pressure, and check resistance (or just swap out) the DME coolant temp sensor.
Fuel pressure at idle should be ~30psi.
DME coolant temp sensor (towards the front under spark plug#1, blue connector) trims the calculated fueling based on coolant temp - much richer for cold starts and leans out as it warms up. They can just get fritzy after many years, and you end up being leaner than you should be which makes for bad power and can be rough idle.

Also, if your idle is fine but just no power when trying to give it gas your AFM might be suspect.

jvandyke

I don't think we bleed the coolant properly yet either so air in there may be confusing the sensor.

Jfrahm

Maybe a cracked intake J boot or a big vacuum leak.

jvandyke

It clears up and runs great though, doesn't repeat until cold again. Odd. Will bleed coolant properly, check connections on O2 sensor and try again. Car will go back into hypernation later today and I need to convince my son to not put it into service again until reasonably sure she's sorted. Momma is raising Cain over the quirky Porsche. It's one thing for the old man to risk driving old quirky cars, another all together for her son.

Pin It

My next step would be to pull the DME temp sensor and check the resistances at a few temperatures, as Spencer mentioned. Not likely to be an O2 sensor issue given that your problem is only when cold...to me that points to the DME temp sensor which is easy to diagnose/replace.

Aivar88

i had similar symptoms when starting cold with vac leak on s2. even when warm, few worst times it didnt want to settle to stable idle, was bouncing ~500 rpm up and down. no power when flooring was common with first cold goes.

snb13

As stated, check the J boot. Could possibly be a broken solder joint somewhere, as well. Everything contracts when cold. When the engine warms up, the culprit could be expanding enough to make solid contact (solder) or close the gap causing the leak (J boot/vacuum.)

jvandyke

Pretty weird. Starts fine, idles good but no matter how long you let it warm up it WILL run on extremely reduced power with in the first half mile, will last from 10-30 seconds, clutch and gently rev, but until it decides to go, it won't go, once it decided, we drove 25 miles and it ran great. Stopped in the middle and restart was fine, no repeat (didn't go cold though)
Coolant bleed and pressure bleed and bleed again. Temp sensor is only a year old. Maybe DME relay beginning to act up? You'd think that would be all or nothing though. We have a spare but it's of unknown history.

944crazy

Not sure if the 924S has the same type of O2 sensor, but I had these symptoms with my 951. Turned out the O2 sensor stopped self heating, and was only warming up from the exhausts gasses. So immediately after startup it ran fine (in cold start enrichment mode) but then after a minute when the dme would switch to closed loop it would run really rough as the O2 sensor hadn't heated itself yet. After a minute or so of running rough, the exhausts would heat the O2 enough to get it running normally again. If your car has a self heating O2 sensor (should be 3 wire sensor) then it might be worth checking this by unplugging the O2 sensor while it's running rough and seeing if it runs normally then.

jvandyke

alright, that's one of the first things I thought of since I know one looked sketchy when disconnected them during clutch work and my old brain can't remember fixing it so I'll go in and check that thoroughly as that makes sense as the problem.

jvandyke

O2 sensor wires check out fine, bummer. Temp sensor wire is still on, hard to get to as you know but it's new (last year). I put in a the "spare" DME relay and the car ran so my kid is going to run that and see what happens. There's an unused FI harness connector floating around under the intake, I think there has always been one thought, wiper on AFM "looks" fine, other connections seem solid, no apparent vac leaks.....
Another run with different DME relay; same result. Warm start, five mile run. Warm restart, 5 mile home; no issue. 1.5 hours later, fired up, idled fine but power loss for several seconds then good again.

Pin It

That unused connector under the intake wouldn't happen to be for the idle stabilizer valve (ISV) would it?

I have also heard of people mixing up the connectors for the ISV and DME temp sensor...I believe they are the same size.

jvandyke

I don't think we have an ISV on this car......
Could still be the O2 sensor, just because the wires are okay doesn't mean it is. I have another I could swap in but would have to hack wires to connect it. Maybe test.........
I'm planning on hooking up the fuel pressure gauge and watching it while doing extensive troubleshooting.
Will pressurize the J boot to check for vac leaks.
I've read one thread where the grounds on the terminal were sketchy, will clean those.
We're still fighting erratic coolant level issue too so there's that.......
I'm thinking idling in the shop and testing all this week, I've got to get this thing sorted and road worthy (it's my son's car) or momma is going to make it disappear. Don't laugh, she can do amazing things when provoked.

jvandyke

okay, yes there's an ISV on this thing and maybe that indeed is where the lost connector should be......will investigate it would be nice if it's that simple