Working on my 1994 940 Turbo wagon. Here is the details as best as I can give.

On cold mornings .. with temps anywhere below 70 degrees .. the car starts but runs bad .. it lopes and skips .. and when it skips you can see the tachometer jump down. After it gets to normal operating temp it is fine ... no troubles at all .. everything is smooth .. but mornings are the worse .. and in the summer months this problem does not show up... below is what has been done with no improvement.

New plugs and wire set
New ECT sensor
New 02 sensor
Throttle body cleaned
Air control motor cleaned
checked all vacuum lines

This car has 260,000 on it and has never had a MAF sensor ... I don't know if that would be the answer and hate to keep guessing... any ideas? I sure would appreciate the help.

 

skipping could be a bad CPS (crank position sensor)... its the wire harness on the CPS that fails, as its tucked in under the back of the head and under the distributor, its hard to see.

the CPS provides all the timing information to the ignition *and* fuel injection. the tach gets its signal off the ignition coil, so it jumping down suggests the timing pulses are missing erratically.

 

Would the CPS only act up in cold weather? .. also when it reaches normal temp none of these symptoms are happening .... only Cold morning starts

 

Would the CPS only act up in cold weather? .. also when it reaches normal temp none of these symptoms are happening .... only Cold morning starts

 

ratty wiring can do all sorts of random things. maybe it only misbehaves when its damp, which tends to come along with cold (from under-the-hood condensation).

 

Pierce - it is not dampness or wet weather that caused it to act up. It seems based on cold weather and start up after the motor temp has gone cold.

It is cold here and I drove to town earlier this afternoon .. came home and car sat for three hours before leaving tonight for church .. went out and it started but it lopes and skips ...tachometer jumping .. once warm to normal temp it runs good.

Do you believe it could still be CPS causing this?

Would MAF cause this?

If not CPS ... what should I look at next?

 

test the ECT (engine coolant temp sensor) at both the ECU and ICU plugs. procedures are on the 7XX/9XX FAQ page on engine sensors, Engine Sensors

all you need to do this is a multimeter with a 'ohms' setting. I would get the engine hot first, then measure the resistance, wait for it to cool down to cold and measure it again. the hot reading should be in the 200-300 ohm range, while the cold reading is in the 3000-6000 ohm range, or even higher if its down to freezing. The ECU is to the right of the passenger side footwell (near the lower front door hinge), while the ICU is behind the dashboard, accessible by removing the driver side kick panel, its above the pedals, tucked into the firewall. I'd find and identify both first before taking it out for the test drive to get it hot, then park it, shut off, and pull the connectors and take the hot readings, wait, and take the cold readings.

 

So you believe there could be faulty wiring? Or faulty icu or ecu?
Should I still replace the CPS?

 

i wouldn't replace anything without testing to determine if its good or bad.

check the CPS wiring. check the ECT resistance hot and cold.

I don't believe I said anything about faulty ICU or ECU, they very rarely fail.

another candidate for intermittently flakey ignition timing (which cuold result in the tach dropping like you say it does) would be the 'power module' that drives the coil.

a vacuum or air or exhaust leak could cause cold running issues. there's lots of candidates.

 

Pierce ... does the MAF play a part of engine control during cold start or does it only take reading from MAF after it is warmed up??

 

[QUOTE=pierce;403773]test the ECT (engine coolant temp sensor) at both the ECU and ICU plugs. procedures are on the 7XX/9XX FAQ page on engine sensors, Engine Sensors

 

let me first give you the ECT readings at the ECU with coolant at approx. 46-50 degrees.

Between pins 13 and 5

1750 ohms (Key off)

3.0 Volts (key on)

Remember this is a 94 940 turbo wagon ... are these specs about correct?

 

that was measured with the ECU unplugged ? I can't see how any volts get in there.

 

that is the measurement done per sensor testing page...

Plugged in key off for ohms test = Cold 1700 ohms Operating Temp = 185.7 ohms

Plugged in Key on for voltage test = Cold 3.0 V Operating temp .470

am I testing it correctly?

Also on my MAF post I tested the results from it.... if you would look them over please

 

ok, I've always measured the resistance at the plug with it unplugged from the ECU, so you're just seeing the sensor and not the ECU... but your values seem in range, so its likely the ECT sensor is good.

I've never messed with voltage tests on an MAF, found the best way to test them is swap with a known good car.

 

Yes I agree that the ect sensor is within range and I would think the ecu is working.

With the test checking voltage on MAF and it not reading what the sensor page said it should that might just be my next new part. I can not find any other pages with test procedures other then swapping it for a known good one.

Would the MAF cause the symptoms I am having?