I have a 92 740 wagon, B230F (rex/regina), with 231K miles that really has a bad intial idle on startup, but then idles OK after a few minutes of running. The car cannot be driven during the rough idle or it will stall or nearly stall. There are no engine codes reported and the TPS is adjusted correctly, and the IAC appears to be working. Also, all ignition wires, distributor cap, and dist. rotor have been replaced without correcting the idle.

Here are some of the symptoms:

1) The rough idle occurs upon starting most of the time, less than ~5% of the time it will idle fine.
2) The idle would smooth out after ~3-5 minutes of idling, but smoothed out quicker after I changed the spark plugs (See below).
3) The idle problem used to only occur during winters in Wisconsin, but now occurs year round
4) The problem occurs with the #4 cyclinder not firing properly; I can unplug the spark wire to #4 with no change in idle

I feel like the problem is electrical in nature because I can "fix" the rough idle by pulling the #4 spark plug wire off about 3/4 inch so that the boot is over the end of the spark plug, but the wire is not seated fully (actually, I could swear that the metal end of the wire is not even touching the metal end of spark plug). I can hold the wire in this postion for about 10 seconds until the idle remains smooth. However, this fix is only temporary until the next time I start the car.

Here is another clue; I changed out all 4 spark plugs last weekend with OEM style NGKs and the engine started and idled fine. The engine also started fine both times the next day. However, a day later the rough idle is back, but only for ~20 seconds this time. Now each time I start the car the idle is rough again on startup, but so far the new duration of the rough idle has been greatly reduced by the new spark plugs.

What can be causing this issues with the #4 cylinder firing? Could it be the alternator, coil pack, or grounds (all original)? Any help would be greatly appreciated as I am getting seriously confused by this.



Thanks in advance,



JTK

 

Next time it does it shut down the engine and pull plug #4 to see if it is wet. Pull plug #3 and compare it just to make sure they both look the same. If #4 is to wet then it might be fouled. If it is to dry then that injector may be either not working or plugged. The only other place to look is the cold start system and the water temp. sensor. Do we also assume you did an ohm test on the #4 sparkplug wire back through the cap?

 

When I changed the plugs over the weekend I did notice that the #4 looked wet and not like the other three (it looked a lot like a 'lead glazed' plug). However, I am leaning away from the problem being with the plugs because the problem persisted even after replacing all of them (unless the new plugs could foul after only 10 miles of driving). I tested the old wires way back in the day and they were OK. I am assuming the new wires (~6 month old Bougicords) are OK too because the rough idle problem is the same as with the old wires. What really confuses me is that I can unplug the #4 wire at the plug, fiddle around with its position and then the idle smooths out within a few seconds, but if I let the idle correct itself on its own it would take a few minutes (now only takes ~20 seconds with the new plugs). Could a bad temp sensor produce these effects?

This weekend I will swap the FI positions and see what happens. I can also try checking the cold start valve (by the way, does this use the same size o-rings as the FIs?).

Thanks for the advice

JTK

 

Do you have any codes?

 

Nope, the condition has never triggered any fault codes.

 

I swapped injectors on cylinders 2 & 4, but no change in the rough idle at startup. The fuel injector in #4 did look more wet than the other injector, enough so that the residual carbon build up on the sides of the injector tip (not on the bottom surface of the tip) could easily be rubbed off.

I also looked into the coil, the contacts all looked good with no corrosion (cleaned them up anyway). I saw a guide on brickboard about how to check for a bad coil, but I could not figure out what they were refering to for "Terminals 1 & 15" and "Terminals 1 & High". My coil only has three terminals, one negative post, one positive post, and a second positive tab. Any Ideas?

http://www.brickboard.com/FAQ/700-90...lureandTesting

The more I think about this rough idle problem the more I can't decide if it is an electric or fuel issue. What I know that both fuel and spark are getting to the #4 cylinder, just that something is off. Any suggestions here regarding the coil, power stage, voltage regulator, temp sensor?

Regards,

JTK

 

New update: I replaced the fuel filter last week and the car has been idling much better than before on start-up. While the rough idle seems to have gone away, I still feel that the problem is not entirely fixed and that there may be merit in looking further into the fuel system. Any thoughts?

JTK

 

I had similar symptoms. As soon as the car would warm up, it ran great. I changed the temperature sensor and had good results. Not a fun sensor to swap out though.

 

Thanks for the tip zloetakoe. I actually have a replacement temp sensor all ready to go. My plan is to change it when I have the intake manifold off as soon as it starts warming up outside. Hopefully, I'll be able to post back in a month or so with a success story.

JTK