As I work my way through two HT4100-equipped cars (and one 4.9-engined vehicle), I have a few more tips for everyone out there:
1. Make sure your heat stove pipe is properly connected to the air cleaner and also the exhaust manifold heat shield (this is only for owners of TBI cars). These sometimes come unconnected/break after the air cleaner gets removed a few times since they're somewhat fragile. While it may seem like you always want cold air flowing through the air cleaner once the engine is warm, the HT4100 is calibrated to run with an intake air temperature of 131 degrees Fahrenheit. Also, check for proper operation of the ThermAC damper door. Make sure it closes when the engine is cold, and opens once the interal air cleaner temperature rises above 131 degrees.
Now that winter has arrived in many states, the heat stove operation is crucial to optimum performance and economy in our vehicles. One of my cars had a slight hesitation when I first depressed the gas pedal. I finally traced it to an unconnected heat stove. Now the hesitation is gone. If you don't believe me that this can be the cause, simply look in your factory service manual under "hesitation" symptoms. I didn't believe it myself.
If you don't have the heat stove on your car (or a loose one), here's what happens: as your car idles, the intake air temperature will rise simply because the engine is hot, and not much air is flowing through the manifold. The IAT sensor will then tell the ECM that the air temperature is rather hot. Then, when you step on the gas to accelerate, very cold air suddenly enters the manifold (as it is sucked in by the engine vacuum). This shocks the IAT sensor, which takes about .5 second to respond to the cold air. Since it was previously telling the ECM that the intake air temperature was hot, this mixture is too lean for the engine, causing a hesitation, until the IAT registers that the intake air temperature is cold, and the ECM adjusts accordingly by richening the mixture.
2. If you haven't advanced your timing to about 12 degrees base (from 10 degrees), do so. If you have a TBI 4.1 or 4.5, simply run mid-grade. You'll experience what feels like a significant power boost, and your gas mileage will also increase. I drive the same route everyday at about 65 mph (about 90% freeway). Before advancing the timing on my new vehicle I got about 22-23 mpg. Now I'm consistently getting 26-27! I'll pay the 10 cents/gallon increase (about 3% larger fuel cost) for a 10+% economy boost and more power.
I just checked the timing on mine via the on-board diagnostics. 17 degrees lol
When I got it back from the last major tune-up it had a thwacking sound but I thought it was just valves slapping from cleaner contact, but now I'm pretty sure that it's the main bearing. Lucas stabilizer got rid of it and gave some compression too, so it hasn't been a problem but ... sheesh ... I'm gonna tell the mechanic to roll it back down to 13 or so. lol
WAIT! You can't use that to check your actual timing. That number will NOT correspond to the actual timing if your base isn't set at 10 degrees. In other words, you'll need to jumper the ALDL connector pins to set the timing in fixed timing mode in order to check the actual timing.
Yes, the 10 degree thing is for setting the timing, but you have to understand that even if you change your timing by 5 degrees (by rotating the distributor), your fuel data center will still read 17. It's the value that the timing SHOULD read if the base timing is set properly. The ALDL connector is behind a cutout in the hush panel beneath the ashtray.
Okay having read some old threads and thought about this some, my guess is that the distributor was probably lined up to TDC without using the 10 degree reference off-set. I'll have the mechanic that did the work double check this and make any changes that are needed to bring it in line, then move the base timing to 13 and see how that goes.
One thing I'm a little curious about is how much to advance the base timing. If you want a 13 degree advance, do you set the base timing to 23 while the ECM thinks it has a 10 degree spark advance (so that when the ECM thinks it is zero it is actually 13), or do you set the base timing to 13 while the ECM has it set at 10?
You set the base timing to 13 with the ALDL pins jumpered (so it's 3 degrees advanced from the base 10 degrees). Personally, I'd set it at 12. When you set it at 13, the No. 1 main bearing usually starts a thumpin'.
With the cold temperatures returning to most of the country, make sure to let your engine warm up for about a minute (before driving) if it's below freezing outside. This will allow the heads/block to expand more slowly than if you were driving it, and put less stress on the head bolts. Also, drive gingerly at least until your heat comes on (usually occurs when the coolant temp hits about 105 degrees Fahrenheit).
If anyone out there has an Olds 350 diesel (I used to own one), this is also good advice!
Automobile(s): 1984 Cadillac Sedan Deville - ht4100
Re: More HT4100 Tips
I have an 84 Cadillac Deville HT4100 that recently started to exhibit the same "hesitation" that is mentioned in the postings here. I checked the heat stove pipe, which indeed was loose, and reconnected it. After taking it for a test drive, the performance seems to have SLIGHTLY improved, but the hesitation is definitely still there.
What else might cause the hesitation thats happening when I push the gas pedal? (I have a Haynes manual, but it doesn't mention anything about hesitation).
I'll be adjusting the engine timing tomorrow (if I can find the distribitor clamp bolt - which I have been unable to do so far ), so hopefully that will help. But if you have any suggestions about what might cause the hesitation and/or slow throttle response, please post them.