1994 Pontiac Grand Prix Shaking at Speed

Do you feel it in the steering wheel?

Does it seem to be more of a misfire or vibration?

The description of your symptom seems more of a wheel balance, or bent rim kind of problem.

i used to feel it in the steering wheel before i replaced the cables but now not so much. its getting to the point where when i started it after work today, it started shaking right away while i was coasting and while i was coming to a stop. i'm pretty sure it has to do with the engine or transmission versus wheel problem because when it shakes the rpms jump up and down in sync with the shaking. also, when it was working perfect a couple of weeks ago i was getting 180 miles per half tank. now i'm down to 140. i had this problem with bad gas mileage before i fixed the cables, then after i fixed them it went up, then the shaking started and now its going down. i'm going crazy thinking everyday my car is going to die on the side of the road. if there is any other imformation i can give to help let me know. thanks so much.

One thing that may help is getting codes retreived from the computer.

If we do have a misfire going on, The very biggset concern on that motor is a leaking intake manifold. They are plastic and have a weak spot. Has your coolant level dropped at all?

in regards to last post, i just drove car around for a little while and watched the rpm pretty closely. when the car chugs while driving the rpm rarely moves up and down. i must have just thought so because it used to always do that before i changed spark plug cables. one thing i probalby should mention is that for about a year and half my car has been burning oil, i usually need to add 2 quarts every one to two thousand miles. i checked the coolant when i replaced cables and it was where it should be. i tried taking it to autozone where they test for free and they said because of the year of my car they couldn't get the codes from it. can a mechanic get codes from it or does a 1994 not have that capability with its computer system?

There are code readers available that can be purchased and most shops have the adapters to read the codes. IT isn't as nice as OBDII but it can help. It's tough to assess over the net without driving it. I wonder if the massive oil burning has contaminated the O2 sensors and or the cat?

yeah i was defianately starting to think it might have something to do with the exhaust. it seems that everytime it chugs the muffler makes a "putt" noise and it gets pretty loud. you think maybe the cat could be the problem? otherwise if the o2 sensor got contaminated what woudl that make the car do?

Hard to say w/o hearing it, but if the cat is plugging up it will run rough and be hard to get power. The O2 sounds more feasible in that it will affect the air/fuel ratio, leaning it out or richening it up. With the gas milage loss, it may be richening the mixure and accounting for the poor economy. That's why it would be nice to have some codes to work with.

where can i take it to get the codes read? an auto mechanic or can i take it somewhere else? i will go and get the codes once i know where to take it and let you know what they are.

You should be able to pick up a code reader at any parts store. It may come in handy in the future.

i'm looking at all these code reader parts and they all say for post 1996 cars. my car is a 1994 without the obd 2. do you possibly have any list of stores or a online website i could buy it from?

Granted I haven't used this one, but you should be able to get the codes out.

OR

You can pull the exhaust down and wire it up, and see if that stops it.

Or if the O2 sensors aren't too bad in cost, replace them. I don't like to just throw parts at something, but sometimes that is the alternative your left with.

Or have a shop diagnosis it.

hey, i just took my car to an auto mechanic my dad and uncle recommended and it seemed like a decent place and the guy was real nice. just want to go over what he said to verify this could be problem. he said cylinder six pretty much wasn't running, there was no oil or gas or anything in there which is causing it to shake. he did a restitance test on the fuel injectors and got around a 12.9 on all of them, i saw this, except for fuel injector six. he did a compression test and got a 120 psi and said 100 psi was lower, or he got 100 psi and 80 was lowest can't remember exactly. he hooked up something to check the codes and there were quite a few of them, except he said they were all from a while ago and no one has been on there to reset them. i did see though it said the gas was running lean. he said he could easily smell sulfur coming out of the exhaust. said that if we replace the fuel injector six, there is good compression in cylinder six, so it should make everything work good agian.. he seemed like he knew hwat he was talking about, but like i said just want a second opinion. should i spend the money to get the fuel injector replaced or do you think there is still going to be somethign else wrong with it?

just wnat to mention to in regard to last post, fuel injector six had a resistance reading of 10.8 compared to the 12.9 and 13.2 the rest of them had. also, my car is the 3100 v6 not 3800 v6 sorry i'm not sure if that changes anything in your diagnosis.

You made a good decision taking it in. Replace the injector as it is out of spec. The 3.1 motors had a fuel injector problem that we often solved with cleaning them professionaly. You can try that, but I expect you'll still need that one anyway.

yeah the guy mentioned the cleaning option but said i woudl probably need a new one anyways. so you think that the fuel injector is the cause of all the problems? is the 12.9 good for the other fuel injectors or should i replace those too? also, is the 120 psi or 100 psi good for the compression is cylinder 6? i'm worried i'm going to pay to fix the fuel injector and find out the whole cylinder is dead or doesn't really work. and one last quesiton, was the code reader saying the enging was running lean because of the dead fuel injector or maybe because of the faulty o2 sensor you mentioned before? thanks again for all your help this forum really is amazing. the quick responses are great you do a really good job volunteering your free time on here to help out. i really appreciate it and i'm sure you help a lot of other people too who really need it. thank you.

I would do a cleaning and see. If it doesn't work, it is still good maintenance and may save the injector replacement. If it doesn't, then replace that injector. The resistance spec is 11.8-12.6. Compression should not be below 100 psi. So that should take care of the problem. The lean code is probably because the injector is not letting enough fuel into the cylinder. There are sensors that pick up the weaker firing on that cylinder and will display that code.

Glad you appreciate it. Some do, some don't...but thats people :)

ok i got the car back the other day and he replaced the fuel injector. it runs about 99.9% better and my gas mileage is way up. i have a 16 gallon tank and last week i got 90 miles to a half tank, i'm still almost on full and i've drove that much already. the only thing that might be a little bit of a concern is when i'm driving at 45 it sometimes starts like its about to violently shake but then never does after the initial jerk. is that just because of the amount of miles or maybe the cat is shot from when the fuel injector was broken? any ideas?

Your kinda back to the drawing board on that. Need to check codes :roll:

How old are the ignition wires?