linderpat

This is apparently a continuing problem from last fall - the same one that kept me from bringing my 928 to Frenzy. I thought I had it fixed. The car is a 78. 71K on the clock. The symptoms are these:

• car fires right up just fine. It drives great, accelerates perfectly and runs very well - for the first 35 miles or so
• when it gets warm. it starts to fade - idle drops, car stumbles and if left to idle, eventually stalls.
• right before it dies, it idles around 100 to 200 - very low. Normal idle is 650 rpm's, and normally very smooth
• after it stalls, it starts back up, but if warm, it will still idle at around 100 rpm or so, and the battery light will flicker. It will eventually stall and die again.
• when it is idling low rpm's, the volt meter on the dash will flick back and forth around 10 volts +/-
• the battery light lights up when the car is started, but then goes out, until the low idle triggers it to come back on, although it comes on weakly - not bright.

Things I've done - last fall it was diagnosed as a voltage regulator problem. I replaced that, and it seemed to fix the issue (the alternator tested fine both before and after the voltage regulator swap). However, I did not drive it enough to see if that really was the problem. Apparently not. Things already done within the past two years:

1. new fuel pump, fuel filter
2. CIS properly balanced
3. plugs, wires, rotor, coil all replaced
4. green wire replaced
5. fuel pump relay replaced

The primary ground strap at the battery is fine. The exciter wire from the 14 pin connector rings fine to the rear of the alternator. I don't know what else to look at. Any thoughts appreciated.

hlee96

I remember reading somewhere that if car has trouble running after certain amount of driving with cis, could it be vapor lock from the carbon canister or rear fuel tank vent being blocked?

accumulator is a problem for restarting after the car is warmed but it doesn’t lead to a stumbling engine while driving right?

Wisconsin Joe

I would tend to think it's not an electrical issue.

Super low idle won't turn the alternator fast enough to generate enough electricity to power the car, so it's basically running off the battery at that point.

As always, not an expert, didn't even sleep in a Holiday Inn Express last night. No warranty express or implied, close cover before striking, contains flammable gas under pressure.

skpyle

What Hoi said about venting. Ed, stupid question, does the level of fuel in the tank make a difference? IE does Blue Maxx act up just as bad on 1/4 tank as a full tank of fuel?

GregBBRD

I'm guessing this is an injection issue, not the charging system.

I'd get it hot and when it is idling slow, I'd turn up the idle speed (to keep it from stalling), adjust the fuel mixture, and then reset the idle speed.

Once you have it running good hot, then you can go back and start it up cold and see how it acts. If it is funky, then you will need to check things like voltage to the warm-up regulator, hot and cold pressures from the warm-up regulator, etc.

skpyle

Ed, another suggestion, try to get a set of gauges on the CIS when it starts to act up and die...
Yes, I know that is blatantly obvious. I am trying to be helpful on a system I know little about.

linderpat

Thanks guys. Good info. Also, occurs with full tank of gas.

SRaouf

On modern cars this is the symptom of a failing Crank Position Sensor. Dies when Hot and starts up again when cold. on My 85 its attached to the rear of the engine just above the flywheel. does the 78 have one?

GregBBRD

No.

Mrmerlin

early cars use the Green wire to figure out when to make the spark

drooman

Hi ed.. I wouldn't assume the CIS is "properly balanced" I'd start with a fuel delivery test when cold, then again after its acting up. If delivery is not up to snuff when hot check voltage at the fuel pump, be sure to check the voltage with the power AND the ground wire for the pump (not chassis ground) also check condition of return hose on top of tank they're usually original. If delivery checks out obtain (or make) CIS pressure gauge arrangement with long fuel hoses, run hoses up and out rear of hood and tape the gauge to your windshield so you can look up from the driver seat and read it. You can drive around this way for quite a while if you make a good quality (safe) fuel hose connections with good quality hose. You can diagnose (or eliminate) slow control pressure creep and system pressure creep this way. I've seen systems that act fine in the shop and then get weird after some run time.

Only after delivery (often assumed and not tested) and pressures are confirmed throughout the temperature range can any tuning be done.

linderpat

Thanks Droo. Considering the black arts aspect of the CIS (at least to me), this one's going to the shop. My guy is one of the few who know these systems and how to tune them.

WESTER928

linderpat, what was the culprit to this issue? I'm having the same problems with my 84 928S. Thanks

Speedtoys

Katies 85 did this.

FL relay would fail when IT got warm.

linderpat

You guys have a different set up. Mine was a k-jet, CIS mechanical fuel injection. The 84 is L-jet, and the 85 is LH I believe. Both of those use a computer and other more sophisticated systems to keep idle in check and the car running at all ranges of temperature.