Wierd Shift

roperninja · December 20, 2009

Hi I just bought a 1997 z71 push button 4x4 with 155k.

Everything seems to be fine I have driven this truck about 400 miles since I bought it with no problems except I can tell it probably needs the intake gasket to be replaced even though the gentleman who sold it to me insisted it has a brand new one.

Anyways,

Over the last couple days I have noticed that at cruising speeds between 40-70, when I put just a little throttle to it I get some wierd jerking feelings. I suppose the best way I can describe it is that it feels like it repeatedly 'wants' to try and kick down and then stops, resulting in some jerky feelings through the cab and hesitation to accelerate. If I let off and cruise slower it goes away instantly and if I increase throttle beyond that point it also disappears and either accelerates normally or kicks down and accelerates as I would think it should.

Anything I should start with or any more information I can give to diagnose? I am looking to go on a 500 mile round trip in this bad boy the day after Christmas and would really rather have this resolved.

Thanks

roperninja · December 20, 2009

Also one more piece of information,

before he sold the truck to me he said the truck was shifting 'hard' into 4th gear at cruising speed but then he 'pushed a plug into the back of the distributor more firmly' and it went away.

mattjulien · December 20, 2009

from what you have said, i think this guy has not been very honest with you. can you give some more info about the intake gaskets? and about the tranny: i would drop the pan, change the filter, and top off with fresh fluid. maybe add some lucas tranny slip in the pan before you put it back on.

roperninja · December 20, 2009

But the transmission always acts just fine under any other situation. Park to reverse is smooth, 1 to 2, 2 to 3, 3 to 4, are all flawless feeling. Its only at that cruising speed and at a certain point of throttle.

I was thinking timing or crank or cam positioning sensors? Maybe something with the spider injection system? Im a former corvette owner turned 4x4 so I am 100% newbie and I promise I am not arguing just giving what input I feel is relevant.

mattjulien · December 20, 2009

you would have diagnostic codes popping up if it were anything you just mentioned, not to mention the engine would be running horribly. the same with "putting the plug into the distributor more firmly". i think it is just the halfway point between crusing and downshift, i mean, how often does that happen when someone normally drives? and how can the intake gasket "probably" need replaced; usually an intake gasket leak starts small and you smell it when you get out of the car and then gets bigger making it obvious that it is the intake gasket.

roperninja · December 20, 2009

you would have diagnostic codes popping up if it were anything you just mentioned, not to mention the engine would be running horribly. the same with "putting the plug into the distributor more firmly". i think it is just the halfway point between crusing and downshift, i mean, how often does that happen when someone normally drives? and how can the intake gasket "probably" need replaced; usually an intake gasket leak starts small and you smell it when you get out of the car and then gets bigger making it obvious that it is the intake gasket.

Regarding the intake gasket -

I found a wet starter with coolant and they said if the right bank was leaking then it would be on the starter and left bank was leaking it would be on the oil filter.

Regarding the shifting -

No possibility of anything electronic? I am unexperienced and just looking for square one.

mattjulien · December 20, 2009

intake gasket-if you really are leaking fluid, it has the potential to become a large leak. this would be a concern for your trip. i changed out my gaskets a while back and no problems since, just make sure you clean everything well and use a very high quality RTV.

tranny-this wouldn't stop your trip. i'm sure it is nothing electrical, but if you would like to, you can unplug all the electrical connections on the tranny and make sure they are clean, and maybe spray them with some sort of electrical lube. changing out the filter and topping off fluid is always good preventative maintainance, it will ensure that you don't have metal caking up in your transmission. if there aren't any huge chunks laying in the pan, i'd say there are no problems with your transmission

roperninja · December 20, 2009

intake gasket-if you really are leaking fluid, it has the potential to become a large leak. this would be a concern for your trip. i changed out my gaskets a while back and no problems since, just make sure you clean everything well and use a very high quality RTV.

tranny-this wouldn't stop your trip. i'm sure it is nothing electrical, but if you would like to, you can unplug all the electrical connections on the tranny and make sure they are clean, and maybe spray them with some sort of electrical lube. changing out the filter and topping off fluid is always good preventative maintainance, it will ensure that you don't have metal caking up in your transmission. if there aren't any huge chunks laying in the pan, i'd say there are no problems with your transmission

Thanks for your replies I will look into that first thing in the morning. I am not new to working on vehicles but will be new to putting a new intake gasket on this particular motor, is there an online instruction leak that we are to go by on this forum and roundabout job time estimate?

mattjulien · December 20, 2009

no prob glad to help. idk about step by step on this forum since i have the manuals! but i'm sure something will come up if you use the search feature. as for the estimated time completion, it took me a whole day to do it, since it was my first time doing it. it really wasn't that bad, just alot of little things that are in your way that you have to take off. darn Vortec motors, tbi and carbs were so much simpler! lol

roperninja · December 20, 2009

no prob glad to help. idk about step by step on this forum since i have the manuals! but i'm sure something will come up if you use the search feature. as for the estimated time completion, it took me a whole day to do it, since it was my first time doing it. it really wasn't that bad, just alot of little things that are in your way that you have to take off. darn Vortec motors, tbi and carbs were so much simpler! lol

Sorry for all the newbie questions but I just got out of a nightmare modded 1993 corvette hot rod and just wanted an awesome stock vehicle with normal car problems (dont ask), and I would like to tend to everything before they add up and or cause problems, I do intend to count on this vehicle to commute about 45 miles a day.

If I do not (which I do hope to) remedy the questionable intake gasket before my long trip is there something I can check every stop to make sure I am not messing up bad?

C6Racer · December 20, 2009

Hi, I hope I'm not too late to provide my input.

Does this ONLY happen between 40-70 mph? What if you put the throttle at that same position at a different speed? Maybe when you take off from a stop. If so, I would check the TPS(throttle position sensor) for an intermittent open circuit. The PCM can't usually detect an intermittent problem, so you wouldn't get a MIL(malfunction indicator lamp), better known as a 'check engine light'. Hopefully you have a scope handy(i.e.: Snap-On Vantage) so you can check the voltage pattern. I'm not sure if you know this, so I'll just say it. The input from the TPS tells the PCM how much you're pressing on the throttle. If there's an open circuit then the PCM can't tell.

While cruising, pressing a little bit harder will unlock the torque converter clutch. It's possible that the valve that controls the fluid flow to it could be getting stuck. A simple change in transmission fluid would likely fix that, but don't do that until you've checked your TPS.

I wish you the best of luck.

roperninja · December 21, 2009

Hi, I hope I'm not too late to provide my input.
Does this ONLY happen between 40-70 mph? What if you put the throttle at that same position at a different speed? Maybe when you take off from a stop. If so, I would check the TPS(throttle position sensor) for an intermittent open circuit. The PCM can't usually detect an intermittent problem, so you wouldn't get a MIL(malfunction indicator lamp), better known as a 'check engine light'. Hopefully you have a scope handy(i.e.: Snap-On Vantage) so you can check the voltage pattern. I'm not sure if you know this, so I'll just say it. The input from the TPS tells the PCM how much you're pressing on the throttle. If there's an open circuit then the PCM can't tell.

While cruising, pressing a little bit harder will unlock the torque converter clutch. It's possible that the valve that controls the fluid flow to it could be getting stuck. A simple change in transmission fluid would likely fix that, but don't do that until you've checked your TPS.

I wish you the best of luck.

Please swallow your milk before reading further less it does not come out your nose...

It turns out that when the light behind the check engine light it out, it becomes hard for the dash lights to alert you of such faults...

I had a 'hunch' to just plug it up at autozone and nailed a 'cam positioning sensor' code, and subsequent 'misfire' code. Im guessing sprang from the bad reading from the cam sensor.

Does this all add up now?

C6Racer · December 21, 2009

Haha, true, a light is pretty hard to see when it doesn't work. I guess the PCM was getting a bad timing input and was providing the incorrect amount of ignition advance, hence the misfiring. I assumed scanning the PCM was the first thing you did, but at least you found the problem. Nice job!

roperninja · December 21, 2009

Haha, true, a light is pretty hard to see when it doesn't work. I guess the PCM was getting a bad timing input and was providing the incorrect amount of ignition advance, hence the misfiring. I assumed scanning the PCM was the first thing you did, but at least you found the problem. Nice job!

Since this will be my first trip into this motor to any extent...Should I replace these intake gaskets whilst at it? Anybody have a roundabout ETA like maybe, could I get it done in 5 or less hours with a helping hand mechanic friend of mine and still make it to work? Assuming no catastrophe (which is likely when it comes to me).

C6Racer · December 21, 2009

If I'm not mistaken, you will have to remove the intake manifold to get to that sensor anyway. A co-worker of mine has replaced the intake gaskets on one of those in about 4 hours himself. Should be cake if you, or someone you know, is very handy and has the right tools.

Good luck!