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Low speed engine surging

12K views 11 replies 6 participants last post by  dodgerunner 
#1 ·
I've got a 97 LHS with 3.5. On the way in to work this morning thought the car seemed to be surging when steady on the gas at a low speed 25-30 mph. This doesn't appear to be a miss but a heavy surge. I looked at my tach and it was bouncing up and down 300 to 500 rpm. Unfortunately or maybe fortunately? I was just about at work when this started to happen - the first part of my drive is on the highway and at higher speeds and didn't notice anything. The idle seems fairly normal, maybe a little rougher than normal but not really noticibly. The wires were changed about a 1,000 km ago and the plugs are the Champion coppers with about 6,000 km on them. Anyone have any ideas where to start looking on this one.
Thanks in advance.
 
#4 ·
Has to do with dirty throttle body valve or idle control valve. Take them both off (just bolt on parts) and clean really good with throttle body cleaner (not carb cleaner). I would also take off the intake plenum and clean out and re-install with a new gasket.
 
#6 ·
Thanks guys. I'll give your suggestions a try hopefully this coming weekend. The temperature is making it a little difficult as the temperature has nosedived in the last week or so and is about -21C with windchill it is -32C which works out to be about about the same in farenheit - too friggin cold. One thing I noticed as well is it is doing the surging thing at higher speeds as well and typically only when you feather the gas pedal or are ever so slightly decelerating.

Thanks
 
#7 ·
Holy crapo - thats -5.8 F.
Defenitly check out the spark plugs and gap them correctly (not .050"). Only use Champion RCL12LYC plugs gapped at .035":. The TSB (#18-18-95) also called for lower viscosity oil if driven below 0C (32F). I think it was 5w-30.

TSB #18-15-95 also called for reprogramming the PCM for vehicles built between Oct. 24, 1994 and Feb. 17, 1995 (MDH 10-24-XX thru 02-17-XX). The TSB gives the symptoms as:

SYMPTOM/CONDITION:
Engine fails to start in extremely cold weather (below 0°F.). Engine will crank and may fire intermittently but does not continue running. Engine may also fail to jump start. It may be necessary to dry spark plugs and move vehicle to a warmer area (repair bay) to start. NOTE :THE FOLLOWING REPAIR PROCEDURE AND PARTS ARE ONLY INTENDED TO IMPROVE COLD STARTING PROBLEMS IN EXTREMELY COLD WEATHER. IT WILL NOT RESOLVE DRIVEABILITY RELATED HARD STARTING OR NO-START COMPLAINTS UNDER OTHER CONDITIONS.

The TSB goes on to describe the repair as reprogramming the TSB and replacing spark plugs (with above part # and gap) as well as changing the oil to 5W-30.

Let us know if this helps.
 
#9 ·
I had a simular problem on my 94 3.5. I got ahold of some bad gas. Once I ran it out of the tank and refilled it, it was fine.
 
#10 ·
Actually I just filled up the tank this morning on the way in to work. Whatever it is, it seems to be broadening it's horizens across the speed range. The car was doing more of the surging at 65-70 mph this morning on the higway. Like I say, I just filled up and it was close to work so I don't know if this is a coincidence but in the couple of miles at a lower speed it didn't seem to doing it. The bad gas may be a real possibility. I guess I'll find out on the way home tonight. I just did an oil change recently and do run the 5w-30 with the cold weather - makes for better lubrication when the temperature is real cold - that oil can get to look like molasses if you don't run a thin enough grade.

Thanks guys. I'll keep my fingers crossed on the way home.
 
#11 ·
Something weird - more info

Well, its not bad gas. Of course that would be too easy. It wasn't bad on the way home yesterday but even worse on the way in today.
Something I noticed the other day and again this morning that is no doubt connected to my problem. I got to my parking spot at work, tried to rev the engine to the rpm range that the surging seems to occur and I couldn't do it. I noticed this the other day but it didn't click until this morning that no matter how slowly or hard I tried, I couldn't get the engine to sit at the rpm range around 1200 to 1800 rpm - under I could and over I could. It's almost like no matter how lightly you push the gas once it hits this rpm range, it jumps to the 1800 + rpms. Also, when you lightly feather it to keep it on the low end of the 1200 rpm scale and let your foot off the gas, the rpms drop down to about 500 to 600 rpm. This is weird because typically when you take your foot off the gas it drops in rpm very slowly down to 1000 or so. It doesn't do this if you get it above that 1800 rpm speed, then it will drop slower down to the 1000 or so rpm.
Does this give anyone any more hints as to the problem?
 
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