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1993 Cadillac STS 170k miles
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51 Posts
Discussion Starter · #1 ·
Hey everyone, tried doing a search but couldn't come across anything.

So here's the deal, while driving up a slight/steep hill while between 45-55 mph the car will start to shake, like lugging. Also getting onto the freeway if I let off to merge into traffic and being between those speeds, then to hit the gas to pick up speed again the car will do the same thing.

If I don't give it enough gas to downshift, the car will start to shake, and will keep shaking with no power till I give it enough gas for the trans to downshift. Once the trans downshifts I have power and the shaking disappears. It only does it between about 45-55 mph.

The speed sensor was replaced like 3 years ago, and I just replaced the throttle position actuator because the car wouldn't stop/idle wouldn't drop(fixed the problem).

This issue has been present through out all these upgrades, I've been putting up with it but now I want to fix it.

Thanks,
Sammy
 

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Super Moderator
2010 DTS
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88,148 Posts
Update your profile to show the year of your car so we know what engine you have. Then pull the DTC's and post the codes along with the code definitions. How many miles on it?
 

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1993 Cadillac STS 170k miles
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51 Posts
Discussion Starter · #3 ·
My bad, I'll update.

1993 seville sts with 160k miles.

I've pulled codes before but there are soo dam many. Its never been reset so it shows the whole history. I use to be up to date with this but I've slacked off. I'll have to go and refresh my memory on how to pull codes and post them up.
 

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Super Moderator
2010 DTS
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88,148 Posts
Look especially at the current codes. If there is a lot, you might want to clear them and see what returns after a few days.

How old are the plugs and wires?
 

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1993 Cadillac STS 170k miles
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51 Posts
Discussion Starter · #5 ·
3 years old. I swapped those when I did the speed sensor. Just had the trans fluid changed and put in a new radiator about a year ago as well. The car runs super good and the engine is really smooth other than this issue I'm having.

So I'll reset the codes and see what comes up after a few days of driving.
 

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1993 Cadillac STS 170k miles
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51 Posts
Discussion Starter · #6 ·
Ok, got the codes. Don't think any that are popping up are the culprit to the problem, could be cause the sensor has not completely faulted yet?

Current:

P040 - Power steering pressure switch open

A047 - Low refrigerant charge


History:

P073 - Intermittent coolant sensor signal

A046 - Low refrigerant charge

T073 - PCM - EBTCM/EBCM Pulse Width Modulation Signal Fault

S044 - Lift or Dive Signal Fault

SO55 - Resistor Module Short to Ground/Open

SO60 - Left Front Position Sensor Fault

SO61 - Right Front Position Sensor Fault
 

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1993 Cadillac STS 170k miles
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51 Posts
Discussion Starter · #8 · (Edited)
Plugs yes, but wires no. To be honest I cant remember what they are.

Should I pull each plug out one at a time and see if they are all firing, your thinking bad plugs or wires?

I remember the speed sensor had a lot to do with the car not downshifting and playing a lot of games with the drivability. But after I did the install of the sensor, plugs and wires I remember it being gone, and the car functioning properly. And how it has slowly progressed to what it is now.

The car idles pretty smoothly, there is a very slight raise and drop in rpm while it is idling, I would say like maybe 100 rpm max. While it does this my lights will dim and get brighter, not much but is noticeable, only while idling though.

Another thing is while getting onto the freeway when the trans is shifting from 2nd to 3rd between 2500 to a little over 3000 rpm the trans will lag going into 3rd gear. Like it takes half a second or so to shift, not instantly like in any other gear. Could just be cause the trans is old though.

Also when the car starts to shake, and if I just keep my foot hitting the same amount of gas, the car will shake quite a bit. Like you could believe that someone driving next to you could see the car shaking. Obviously till I give it enough gas for it to downshift, then its completely gone.

Trying to fill you in as much as I can about how the car is running, give you a better idea.
 

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Super Moderator
2010 DTS
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88,148 Posts
Hard to say if it is plugs or wires for sure, but it does sounds a little like it. Even a weak coil could do that. Hate for you to start throwing parts at it though. That can get expensive.

I doubt it's the fuel filter. You say it runs OK when it downshifts. If the filter was clogged, that's when it would starve for fuel, at higher RPM and fuel demands.
 

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1993 Cadillac STS 170k miles
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51 Posts
Discussion Starter · #11 ·
The way I see it is like a manual trans; when you first let out the clutch and if your not giving it enough gas the car will lurch. Also when your coming to a stop and you dont push the clutch in, the car will lurch. But at the same time if your on the freeway with the manual trans and put it into a pretty high gear, lets say 6th, the car will basically idle till you give it gas, then it will sput a little and you will absolutely go no where. But the car doesnt really shake like what I'm experiencing.

So saying that, if your in a too high of a gear for your speed, the car should just have loss of power, like with a manual. So it should just flood the engine with fuel and air, not shake the crud out of my car. Because the trans is what puts the car into its necessary power band, so its basically the shaking what I don't understand.

But then the N* power is mostly up high, not low like a small block v8. So maybe you could get away with lugging a LS1 compared to the N*. But when you truly think about it, if a car is not in the right power gear for your speed, it will sput a little when you first give it gas, then you will be fine. But you will be giving it gas but not moving anywhere. My car doesnt move either, but shakes quite a bit.

Its almost like there is some sensor not tell the fuel supply to pump more fuel. I have a throttle position sensor laying around from the last time I went to the junkyard to get my throttle position actuator. So ill put that on and see if anything changes.

Today when I leave I will try and barely give it gas each time while driving on the street or freeway and see if it does it at other speeds as well.

Thanks for the continuing insight.
 

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Super Moderator
2010 DTS
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88,148 Posts
Its almost like there is some sensor not tell the fuel supply to pump more fuel.
That is the job of the MAF sensor for one. MAP and TPS also have inputs, but you have no DTC's on any of these.

Have you verified that the fuel pressure is good and the FPR is not leaking?
 

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1995 Seville SLS, 1980 Corvette
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46 Posts
It sounds EXACTLY like what mine did earlier this winter, and at the suggestion of Ranger and others here I did the plugs and wires (AC Delco) and it was solved.

I had a few plugs that had virtually no electrode left at all resulting in a gap almost double the spec and very dry and brittle wires.
 

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1993 Cadillac STS 170k miles
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51 Posts
Discussion Starter · #14 ·
Ok, well before I left today I went out to check the plugs and wires...

and what do you know the back passenger side spark plug wire was not plugged in, and was sticking about an inch out of the plug hole. I looked inside and it looks like the top of the spark plug has some rust on it, from moisture or something. So thats good proof that it hasn't been plugged in for quite some time. So after discovering this I decided to hold off on the TPS and see if that was the culprit. After getting on the freeway I tried pretty hard to get it to lug, and it did do it but to where it was barely noticeable, like 80 percent better. Really paying attention to the car and driving on the freeway I tried giving it barely any gas at any speed, and have know noticed that it does it very little, even at higher speeds(65-70mph). I was thinking this whole time it was a sensor, and basically came down to some stupid plugs/wires.

So I guess this could be the sign to what the problem is, the spark plug had rust on the top of the plug so I'm pretty sure there still isnt a very good connection to the plug. So this weekend I'll take the spark plugs out and check them, and at the same time inspect the wires more meticulously. I was never thinking spark plugs/wires cause I just did them 3 years ago for about 35k miles, and my dad has changed them only twice on his 2000 astro van and he has 250k miles on it.

I don't let anyone work on my car and always do everything myself, except for oil changes. So someone at jiffy lube must have been snooping around my engine and accidently hit the plug just enough to make it come loose.

Any suggestions on who sells acdelco plugs/wires at a good price?

Thanks Ranger for sticking it out and helping me. I'll update the thread when I have the chance to check the plugs/wires hopefully this week some time.
 

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1993 Cadillac STS 170k miles
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51 Posts
Discussion Starter · #16 ·
Ok sweet. Do I get platinum or double platinum?

Hard to find that thread with all the ignition info.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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73,446 Posts
Ok sweet. Do I get platinum or double platinum?

Hard to find that thread with all the ignition info.
I believe your engine (4.6L Northstar?) takes the ACDelco # 41-950 Platinum plug (they're double platinum but RockAuto lists them as single) pregapped to .050".

Talk to Chris up in RippyParts ^^^ or www.rockauto.com.
 

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2000 DHS
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24 Posts
Sammy22...
Hello, I have been trying to describe the same thing you had... mine doesnt shake horribly but you can tell it doesnt sound good and the vibration is bothersome. So how did it go after replacing plugs and wires?
I would really like to know your results.... i will be checking my codes sat.....

thanks in advance
 

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1993 Cadillac STS 170k miles
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51 Posts
Discussion Starter · #19 ·
^Greggy18

Hi. Yes, by changing the plugs it has fixed the problem. One thing I would do just to make sure its the plugs or wires is to take the plug out; leave it connected to the wire; and start the car and make sure they spark. Only do this if your plugs and wires have not been replaced for a while; if not then just replace them because they have to be replaced anyways. But if that doesnt solve the problem then it could be an ignition coil. Most likely its the plugs though.

Does the trans shift fine, and isnt acting up?
 

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2000 DHS
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24 Posts
Thanks 4 your response!!!!
don't think I can try the wire with the spark plug out because mine is a 2000 dhs and has no wire other then the boot the connects the coil to the spark plug. I'm going to replace the spark plug and most likely the boot also. like I said my problem wasnt quite as severe as you're shaking but the symptoms are the same.
as far as transmission shifting, super smooth.... no jerking no hard shifting..... you gotta love that northstar power !!!! I will let you know what happens if you're interested.
 
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