Hey,

I'm new here, and have searched through the forums to try and get an answer, but didn't find anything for this specific set of circumstances.

Here's the situation:

This is actually my girlfriend's car, a 1996 Blazer with Automatic transmission and the 4.3 motor.

The car started having ignition issues a month or so ago, where it would miss and hesitate on acceleration and idle would be a little rough. I went and replaced all the plug wires, cap, and rotor. The cap and rotor were obviously in need of replacement, as there was a lot of build-up on the contacts there. While the car ran better, the missing did not go away. I took the car to AutoZone and had them do a readout (as the SES light was triggered by the problems) and got a bad Cam Position Sensor reading. I replaced the sensor, and after this, the missing on acceleration and idle went away. I had reset the computer by removing power, and let it go a while (100 miles or so) so it could acclimate and everything seemed fine.

The car is mainly used for in-town driving (she works only a couple of miles from home via city streets), but we noticed that on trips across town via the highway, we were getting a shutter in very specific conditions. I had noticed this another time shortly prior to the acceleration/idle issues, and when those problems surfaced, i thought it was all the same problem. Now that those issues have been resolved, i think this was a separate problem, as it continues to occur and is more frequent.

What's happening now is that during highway driving, the car seems to shutter only when cruising at about 60mph, with the rpms right around 2000rpm. It does not happen at slower or faster speeds, or when going uphill at 60mph, when the transmission downshifts to maintain speed. Only then does the SES light come on, and will go away when driving in town after a while. I took the car out on the freeway today to get the light to come on, then took it to Autozone again to get the reading. This time, we got a multiple misfire error (P0300), which is vague enough not to have an obvious solution. Suggested fixes were ignition component replacement (most of which I've already done), including the coil. I doubt this is a coil issue, as it ONLY happens at 60mph/2000rpm, and I'd imagine a bad coil would exhibit issues at other times. Other suggested cuases were a vacuum leak, injector fault, or fuel pressure. None of these seem like they would fit the issue, as they would happen at higher RPMs (such as the fuel pressure issue) or at the same RPMs in different gears.

I'm leaning towards something drivetrain-related, and my research landed me on an interesting possibility; the vehicle speed sensor. I found a post found here: http://en.allexperts.com/q/Chevrolet.../f_4247641.htm that may suggest the speed sensor is going bad, and causing torque converter issues, causing the shuttering. His description of the problem is identical to mine, so I'm inclined to think that this may indeed be the problem. The shuttering itself feels to me more like a drivetrain issue than ignition-related as it is not as erratic as earlier problems, but more of a back-and-forth feel, so this theory feels right to me. My only hesitation is that I would think that the speedometer would be going nuts if this were the case, and the speedometer works just fine. I'm making the assumption that the speedometer pulls its information from this same sensor. If this isn't correct, then the fix is quite feasible.

Can anyone help clear this up for me?

 

A TCC shudder issue will not set a P0300 code unless it is violent enough to trigger the knock sensor. That would have to be quite violent to do that. You could try removing the VSS and cleaning it. Sometimes they get dirty and have erratic readings. This would, however, also cause your speedometer to jump around if the VSS was truely the cause.

Also, typical TCC shudder concerns happen around 45MPH which happens to coinside with converter lockup during normal cruising.

If you have not checked over those things you have mentioned, you may be overlooking the potential cause for the problem. For instance, the coil and ICM can have heat induced problems which is related to the duration and frequency of the spark. Quick stints through the gears may not produce the heat that running steady at 2000RPM might.

I will add to those things which you have not checked already with checking the TPS sensor voltage. It should measure out to be ~0.5V at idle and smoothly increase to ~4.5V at wide open throttle as you open the throttle blade. I have seen these wear in one spot. This happened on my old 2000 Bravada and caused a pronounced miss while cruising around 65-70MPH.

Another thing that may be worth checking into if everything else specs out fine is the gear on the bottom of the distributor. Sometimes this can cause erratic spark timing. While it is often more pronounced under heavy loads, light loads misfires can also result.

 

Thanks for the response.

As an update, I found that it does demonstrate the same problem at a lower speed, and I had not observed it until yesterday during slow acceleration. At lower speeds (around 30-ish), it happens at more like 2300-2500 rpm.

The TPS having a bad spot makes a lot of sense, since it is consistent in relation to engine speed in a specific gear. The same throttle position would result in a higher rpm at lower speed (less load), so it makes sense that it would happen the way it is.

Also, I failed to mention before, the car has over 196K miles on it.

 

So it wasn't the TPS. I replaced that, and the issue continues. I'm going to replace the plugs next since they weren't replaced before, and I'm not certain the last time they were replaced. Next after that, the coil, then the ICM.

We already had the plug wires on-hand before when I was initially troubleshooting it, and I replaced the cap and rotor because the cap broke when I was pulling the plug wires from it. I'd had her grab those on the way back to the house as I was working on it, and forgot to tell her to get new plugs. Replacing the wires themselves that day was such an ordeal (I still have the scratches healing on my forearm from getting my hands down in there on the passenger side of the motor to pull them) that I knew I wouldn't have the time to replace them anyway that day, so I never went and got them then.

Am I the only one who thinks this motor wasn't designed to be easily serviced? Getting to things that are supposed to be regularly maintained is real pain (plug wires, plugs, distributor cap/rotor, oil filter, etc.). I had to go and specifically buy a short T15 driver just to get the distributor cap out. It's like working on a mazda car. The only time I've been pleasantly surprised with any sort of maintenance work on it was replacing the serpentine belt.

I checked the vacuum hoses while I was replacing the TCM, and they all look to be in good condition.

Barring any of that fixing the issue, I suppose I can pull the distributor and check the gears. I doubt that's the issue, as it doesn't occur under hard acceleration, where you would naturally think it would be most pronounced. Also, it's a real pain to get to, so I'd rather not pull it if I don't have to.

Thanks for the help and suggestions.

 

If by regularly maintained you mean every 75-100k...

I really did not find it difficult to maintain. I mean it is no where near as easy as my 1974 K5 is to do a plugs, wires, cap, & rotor on, but it is a lot easier than most other vehicles I have worked on in the past.

I am sorry that you wasted your money replacing something that you could have tested as suggested. It is frustrating when suggestions get taken as "replace the TPS" instead of the test I intended... If you want to throw parts at the problem, that is your prerogative. Inspecting and testing components would be much easier on your wallet. All it takes is time and patience.

 

Perhaps I chose my words poorly. "Routine" maintenance (which I would consider replacing spark plugs/wires to be) is the term I intended. Perhaps you have considerably smaller arms than I, as I can't pull plug wires from the passenger side without suffering light injury. Pulling the distributor cap and rotor involves standing on the front bumper to get sufficient reach, as it's buried at the back of the engine compartment. I've worked on many cars, and can think of only a few where I've had such trouble with minor parts replacement like this, my '75 G30 being one of them. Perhaps it's a Chevy thing to make the spark plugs inaccessible.

I didn't take your advice as meaning "replace the part", however I have no idea how to check voltage on a sensor that's got a sealed plug while it's working, and do not possess a computer reader to check things like that. I replaced the part I thought most likely to be the issue, and I was wrong. It's not a big deal. And it's past time for the plugs to be replaced anyway (apparently they've never been changed since she bought it 100K miles ago), so there's no skin off my nose there.

Incidentally, the car has stopped missing. It was occurring immediately after I replaced the TPS, so I know it's not "fixed", but has gone back to being intermittent and latent. New plugs go in tomorrow, so unless I find one that's obviously damaged/worn, I'm just going to have to wait for it to come back or decide after a long enough period of running properly that it's fixed.

 

Updating here because the problem has been ongoing, albiet intermittent until more recently.

I had the ICM checked and have replaced the coil, to no avail. It was suggested to me to run some injector cleaner through the car, so I have since run a tank with Lucas Fuel Injector Cleaner, and a second tank with SeaFoam.

At first, it seems that these had helped, but the problems returned again over time, and it seems to correlate with the fuel tank level. The fuel pump was replaced less than 10K miles ago, so I don't think that it would be the issue, so my guess is that the problem is something in the fuel regulation under the hood.

In the time since my last post, the problem has worsened considerably, to the point where normal acceleration shudders and misses significantly at low speeds as well and is increasing in severity.

My best guesses at this point are perhaps a bad Fuel Injector Pressure Regulator or the Electronic Control Unit itself. If anyone has any suggestions, they are much appreciated!

 

you haven't worked on to many new cars if you think tuning up a 2nd gen s10 is hard but i guess if it was easy everybody would do it and i would be out of a job.The distributor is junk in the 96 and up vortecs and is very comin to cause problems like this that is what i would sujest