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Discussion Starter · #1 ·
Here's one that's got me scratching my head about my 96 Roadmaster Wagon...

The car seems to run perfectly unloaded. It has received no major repairs in the last 40,000 miles — just typical service items.

The last 4ish times I've towed a boat, at the 10 mile mark it begins to run poorly. It seems to be down on power and will pop and sputter a bit on idle. It has happened while towing two different loads (6000lbs and maybe 3500lbs). I've owned the car for 4 years and have never had this issue before.

The car has made six 250 mile trips and has been daily driven between episodes of poor performance while towing, so I'm convinced it's only triggered when burdened by an additional load. It seems to happen at the same distance from departure each time.

Any thoughts? My first thoughts go to engine temp, O2 sensor, or transmission issues. Temp gauge reads normal when this occurs, but I know they're not known for their accuracy. No codes are thrown, but I was wondering if the additional load was affecting fuel mixture/O2 sensor input. Transmission doesn't feel like it's slipping, and the weird idle wouldn't be explained by this.
 

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What rear axle gear do you have? 2.56? 2.93?
(What mods, if any, does your car have?)

When you tow, do you tow in 'D' or 'OD?

Has your pcm been reprogrammed?

When was your last ATF change?
Are you using Dexron VI now?
 

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I'd start with the PCM's engine temperature gauge. The dash gauge temperature sensor is different from the one in the water pump that feeds the PCM. Get a datalogger and monitor what the PCM is being told.
 

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Discussion Starter · #4 ·
What rear axle gear do you have? 2.56? 2.93?
(What mods, if any, does your car have?)
2.93 - no mods

When you tow, do you tow in 'D' or 'OD?
No OD towing...well occasionally with an empty trailer, but I've never had this problem while towing empty.

Has your pcm been reprogrammed?
Nope.

When was your last ATF change?
Are you using Dexron VI now?
To this I have no answer. It would probably be wise to knock this out soon.
 

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Discussion Starter · #5 ·
I'd start with the PCM's engine temperature gauge. The dash gauge temperature sensor is different from the one in the water pump that feeds the PCM. Get a datalogger and monitor what the PCM is being told.
If I'm not mistaken, I think the temp displayed on the climate control in diagnostics mode is what is read by the PCM. I kind of forgot about that. I'll have to pull that up next time and see how high it gets
 

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You'll probably think this is not helpful, but: 3.73.
Maybe you'll consider upgrading your KD1 ATF cooler?
Drop the pan, let the old ATF drain, replace with Dexron VI sooner rather than later.

The dash temp gauge senDER & the H2Opump temp senSOR do not always necessarily correlate.
Using the climate control diagnostic mode to track the H2Opump temp sensor will show you that real quick.

Merely avoiding 4th gear by putting the shifter in 'D' is not the same as programming the pcm to avoid early upshifts and reluctant downshifts.

(If I ever wind up with a V08 mech fan, I'll switch the metal mech fan for a plastic one.)
 

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Discussion Starter · #7 ·
Welp. Made the same trip today with the same load and had no issues. Ambient temp was about 5 degrees higher.

I ran the diagnostics mode on coolant temp the whole time. It mostly ran in the 210-219 range and briefly crept up to 222.

You'll probably think this is not helpful, but: 3.73.
Maybe you'll consider upgrading your KD1 ATF cooler?
Drop the pan, let the old ATF drain, replace with Dexron VI sooner rather than later.
Yeah, I'll end up with 3.42 or 3.73 at some point, but that'll be a part of a larger rebuild. I'm mostly trying to find this gremlin because I'm moving about 6 hours away, and I've been planning on transporting my 50 Chevy pickup on a trailer towed by the RMW. I'm too busy to mess with it now, but maybe I'll drop the wagon off to have the transmission flushed and filter replaced before the move.
 

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regardless of the cars running condition while towing...you are killing it with the 2:93 gear. Get 3:42 minimum and strongly consider a larger external tranny cooler
 

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Fuel filter?
 

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Gears, water temp, fuel filter has nothing to do with it!! I have a 95 Impala SS that has the same issue. I replaced the ignition wires, cap and rotor and fuel filter, spark plugs are good. Still runs horrible. My problem is intermittent. At idle it revs all the way to 6,000 rpm in 1.5 seconds but when you load the transmission the engine runs bad. It has no power. It runs better with 93 octane but still slow to accelerate rpm’s. Transmission shifts hard at high rpm’s no slippage. Hmmmm....
 

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A. If your rear axle is not 3.42 or more, you MUST tow in 'D' to avoid 4th.
Over-run clutches engaging on every throttle lift = ATF is gonna heat up more than it would in 'OD'.

B. GM may have rated LT1 B-cars to tow 5000lb provided the shifter stays in 'D'.
However, the 4L60E will still upshift early to 2nd & 1st, and will not downshift to 2nd or 1st without enough throttle.
The OEM shift strategy is still trying to get 17MpG metro-urban. (It will fail even moreso towing extra weight.)
Too many early upshifts & reluctant downshifts + extra weight = ATF is gonna heat up some more due to the extra weight.

C. OEM fan-on threshold for the 2ndary electrical fan is 232°F. That's engine coolant temp, NOT ATF temp.
You can bet money that if two otherwise identical B- or D- cars are both running engine coolant temps of 232°F or more,
the ATF is hotter in the one towing, than in the unladen one.

Deez nyutz
Bonus info: when the coolant temps reach or exceed 219°F / 104°C, the OEM program applies at least 7° of BurstKnock.
7° of BurstKnock makes for sluggish throttle response. (Also, your ATF is hotter than need be.)
If the coolant temp is 219°F / 104°C or higher, 7° of BurstKnock may not be such a bad idea, especially if towing.

Solutions:
1. Improve airflow to the KD1 ATF cooler behind the passenger side of the bumper.
Cut a hole in the bumper fascia (Olds Custom Cruiser front bumper faces are extinct), or add a fan to the cooler.
Better yet, upgrade to a superior ATF cooler. Better still, all of the above.

2. Shifting it yourself might prevent excessive early upshifts.
However it takes a bit of skill and timing to downshift properly, especially when towing.
A better solution would be to reprogram the shift strategy for when the car is towing to reduce or eliminate the early upshifts & hesitant downshifts. The more often the unnecessary shifts, the sooner the rebuild.
(The sexy solution would be to convince Gary @ Innovative Wiring to make a Tow/Haul Mode Switch & Wiring Harness for our cars, but for the time being, he seems to be hesitant to respond …)

3. The 2ndary fan should be reprogrammed to turn on no hotter than 218°F.
Most LT1 tuners prefer the 2ndary fan turn on by 212°F.
Obviously this is in conjunction with the better shift strategy and other underappreciated tune features.
 
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