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940 Intermittent Rough Running on Start-up.

740Weapon

Traitor
Joined
Aug 2, 2004
Location
Salem MA
Hello everyone,

I am having a long-time issue with my 940 that I am having trouble sorting out. A board member is very interested in the car so I would like to get the communities advice on fixing a long-standing annoyance.

Sometimes (very intermittent) on start-up I have a dead cylinder (#3 I think based on unplugging fuel injectors this one time, need to re-re confirm) for about a minute or less. Issue seems to happen only when the temperature is below about 45 F. Here is a list of things I have done:

No trouble codes. Just the default 1-1-1 for fuel and EZK.

New ignition coil
New ignition amplifier
New plugs, wires, cap, rotor within 10k miles. New plug and wire on Cylinder #3 twice. I have checked resistances on the distributor, rotor, and wires and it all looks good.

Did a fuel-system leak-down test and lost very little pressure over the 20 minutes of the test. (well within spec)
Visually, injectors look very clean and they have been professionally serviced within last 6 years or so (Brown top injectors). Resistance on all injectors shows normal, wiring grounds and pins are clean.

Resistor pack is visually in good condition but I have not measured the pin resistances to confirm.

Resistance of the temp-sensor shows normal, e-fan function is normal, and wide-band oxygen sensor warm-up measurements seem typical. I think that the temp sensor is fine. (I have not checked at the ECU pin.)

Engine compression is fine; all cylinders are about 150 psi.

I have gone from narrow band O2 sensor emulation to running an actual narrow-band O2 sensor.
When running rough, I disconnected the O2 sensor once and the engine immediately smoothed out. (IDK if this is coincidence; I have not tried enough times.)

Issue can happen on overnight cold start and a luke-warm start. I have not seen anything weird happen after the car sits a few days or for hot-start.

Oil consumption habits have not changed. (Proportional to throttle use)
Coolant levels have not changed.
No funny gasses leave the tailpipe when the car is running rough.

When the motor starts running well, it’s more like an on-off switch, not a gradual smooth-out. The car runs very smooth and very fast once the cylinder catches so I don’t think its weak ignition, a clogged injector, or dying engine. Also, if I go into boost while running rough, this seems to speed-up the smooth out.

The wide band oxygen sensor has not shown anything out of ordinary when running rough.

I’m having trouble getting a consistent story out of the things I have tried.

Potential next steps are:
Check resistance of radio suppression relay after powering it
Check resistance of injector resistor pack pins
Re-re confirm that cylinder #3 is the trouble maker
Swap #1 and #3 injectors and see if issue follows

Added experiments thanks to this thread:
Try stock spark plugs
Try stock fuel injectors


What else?



I’m thinking that the 3# injector is just sticky in the morning. I run only shell gasoline and have run several bottles of fuel injection cleaner from various brands through the system without a change. I know that brown-top fuel injectors are not known for their longevity. That being the case, what are some good fuel injectors with a similar flow-rate these days?

At this point, I am looking to make a big list of things that myself or the next owner could do to sort out the rough-running problem.

I appreciate any help I can get, Thanks,
-Dana
 
Last edited:
I had a problem like your with E3 spark plugs.

Interesting. I am running NGK copper spark plugs two heat ranges cold for the high-boost situations gapped 0.028.

I will add 'try stock heat-range spark plugs' and 'try stock injectors' to the to-do-list.
 
I have tried a lot of different plug in search of a few mpg. I run ac rapid fire #4. The e3 came from advance and they refund my money. My car are N/A, and both get 27 mpg hay with the aw70. Going to a aw71L in the 87 in the spring.
I went through a lot looking for my problem and the plugs were the last thing because they were new.
 
Rsr

i suggest to bypass radio supression relay as its a easy thing to do.
also i second the swapping out injectors to rule out bad one/ones.
good luck
 
How did the car drive when the problem was occuring? Or do you even attempt to drive it?

I had an odd problem start this morning, sounds similar, but probably not the same. '92 240, LH31. converted to LH2.4, and +T'd...lots of possible issues introduced with these changes, but it's been reliable for over a year, so something has gone awry.

Car started fine, idles low when cold because the IAC is bypassed and idle set with the throttle stop. Works fine for me. But what's strange is the car stalled as I was about to take off from a stop. Restarted very easily. Take off from a stop and it barely goes. doesn't want to idle at all, just dies if I let off the throttle. Still revs, kinda sluggish off idle, but if I give a lot of throttle quickly, it revs up to 2-3k no problem. OK, I can drive it like this.

Start driving and it's very sluggish at low RPM, low load. If I rev it up and then take off quickly, it's better. Give a lot of throttle and get into boost a little and it's almost normal, but try to just crack the throttle open to maintain cruise and it just wants to fall flat and not accelerate. It never felt like a dead cylinder (just had that on the '87, very rough idle), but it did feel like a misfire or slight hesitation.

After I stopped to check under the hood for anything obvious, I continued driving and it suddenly went away as I was pulling out of the parking lot. Drove another ~3-4 miles to work, no problem, idle was solid at ~1000 as it usually is once warmed up.
 
How did the car drive when the problem was occuring? Or do you even attempt to drive it?

Hi Mike,

The problem with my 940 is very similar to what you described. It however does not stall, probably because the IAC is functional.

I have not tried anything since making my original post because the car is currently stored for winter.

I would like to add that my 740 16 valve has had a similar problem for many years. The big difference is that the 740 runs on seemingly 3 cylinders for a minute once the weather falls near 0 degrees F and the 940 seemed to act up once weather gets near freezing.

If you find a solution, please post it.

-Dana
 
Found the blue/white wire to the TPS wasn't connected (should be power from ECU). But it looks like it was never hooked up after the swap from 3.1 to 2.4. Unplugged the TPS, no change. Don't think that's it.

Swapped in a junkyard AMM that I picked up last weekend. Ran a LOT worse. Similar to limp mode where it would not take quick throttle input and it was very sluggish. AMM might be bad that I swapped in? Swapped the original AMM back in, and the car is drivable again. Still has the dead spot around idle to 1500, but it's not all the time.

Still need more time to drive it and try to pinpoint the issue.
 
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