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Old 08-10-2007, 07:37 PM   #1
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What might be causing this?

From a dead stop, with anything over 2000 RPM, my MH either studders or seriously vibrates up through the peddle and through the seat. If I let up on the accellerator, it smooths out.

Additionally, when traveling at a higher speed around 60, if I push down the pedal, it feels like the whole rig is bouncing. When I let off it smooths out.

I have only driven 300 miles since I bought this MH. It has 15,000 miles on it. When I test drove it and brought it home, this never happened and I tested it hard.

I am taking it back to the shop again and want to get some other opinions as well.

Thanks in advance.

gw
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Old 08-10-2007, 07:37 PM   #2
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What might be causing this?

From a dead stop, with anything over 2000 RPM, my MH either studders or seriously vibrates up through the peddle and through the seat. If I let up on the accellerator, it smooths out.

Additionally, when traveling at a higher speed around 60, if I push down the pedal, it feels like the whole rig is bouncing. When I let off it smooths out.

I have only driven 300 miles since I bought this MH. It has 15,000 miles on it. When I test drove it and brought it home, this never happened and I tested it hard.

I am taking it back to the shop again and want to get some other opinions as well.

Thanks in advance.

gw
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Old 08-11-2007, 04:03 AM   #3
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Well.. I can tell you how to make it accelerate from zero to sixty smooth as silk.

Just put a mechanic in the shotgun seat
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Old 08-11-2007, 05:36 AM   #4
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I hear that! (I should have married a mechanic)
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Old 08-11-2007, 06:32 AM   #5
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Check the obvious things first... like the plug wires and plugs, fuel rail pressure etc.
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Old 08-11-2007, 10:14 AM   #6
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Well, here is an update.

I just got back from taking it to the dealer. The journey is 80 miles, so I had a lot of time to recreate the symptoms. By the time I got there I could place it on cruise control at 62mph and watch it do the shudder/vibration everytime the cc accelerated a little.

This is what I witnessed at least 50 times consistently:

Cruising smoothly at any speed, accelerate, shudder (3 to 5 seconds), downshift, pick up speed, cruise smoothly.

Everytime the engine needed a little more fuel to maintain the speed I set on the cc, it shuddered violently as it took the fuel, then dropped a gear to get past the shudder and then it would smooth out again. This created a very rough ride with lots of down shifting and revving on the ride to the shop. Something is definitely not right there.

Without the cc on, I could press the accellerator at any cruising speed and get the shudder. I could keep it in the shudder zone as long as I wanted to, or press harder to get over the shudder and smooth out the ride.

As a correction to my previous post; from a dead stop it shudders as I accellerate between 2500 and 3000 rpm. Any thing greater or less than that range is pretty smooth, but has a small shudder.

It only shudders on acceleration. The coach rolls smoothly at all speeds on decelleration.

The engine runs great at all speeds, doesn't sputter or backfire, or hesitate at all. It seems like there is an issue with the transfer of power to the drive train, not so much an issue with the engine. This problem is even more apparent when I put a 2000lb trailer behind it.

So, that is the best I can describe it right now. The shop is closed today, so I won't know what they think or if they can recreate the problem until Monday, but I sure would appreciate any other opinions on this.

Thanks.
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Old 08-11-2007, 02:53 PM   #7
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Sounds like you are dropping a cylinder on acceleration. Most probable cause is spark plug wire, or spark plug. Your dealer should be able to determine this very easily. Good luck with the repairs, and I hope it is as simple as this.



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Old 08-12-2007, 04:11 AM   #8
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Although I know there are spark plug wire issues with some WH engines, the engine never faulters. Spark issues would seem to cause a lot more symptoms than I am experiencing, so I am inclined to think more along the drive train. I have had a similar experience in two other cases on much different vehicles.

I broke a motor mount and it created the same effect as described above. The torque of the engine, broke a mount, which from that point forward, gave the transmission a different shift point based on how mild or hard you accelerate. Mild accelaration caused the same shudder.

I have seen where a slightly worn U-joint picked up a shudder on mild acceleration, but ran smooth on deceleration.

Anyone have similar drive train or mount issues with the Work Horse chassis?

The only other thought I have is a possible vacuum issue. Where a cracked or loose vacuum line may be sending mixed messages to the transmission.

I know the dealer could have a lot of other ideas, but I would like to get this back as soon as possible, so any input from those that have had a similar situation would be helpful.

Thanks again.
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Old 08-12-2007, 05:21 AM   #9
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I too think you may have a single cylinder misfire probably due to a burnt plug wire.

The 8.1 is a powerful engine and even when misfiring on 1 cyl. you might not know it until the transmission torque converter clutch is applied or you accelerate HARD from a stop.

You give an excellent description of the scenario I have encountered MANY times on other vehicles as an auto/truck technician. More often than not the customer perceives the issue as a trans. or driveline problem.

Since you say the problem happens on hard accel. from a stop AND at highway speed on light accel I suspect a misfire is occurring. It is under these two conditions when cylinder pressures are highest and consequently the demand for spark is greatest. If the spark can't jump the plug gap, it'll arc out of the wire to ground. When the torque converter clutch (AKA the lock-up clutch) is applied cruising at highway speed and you start up a slight incline or accelerate lightly this can cause a misfire that is VERY noticeable and may even shake or vibrate the whole vehicle because the engine and trans. are coupled together by the lock-up clutch. If you release the throttle or apply the throttle enough, the lock-up clutch turns off, torque multiplication is increased through the torque converter and engine load drops which may eliminate the misfire temporarily. Even if the misfire is still occuring the fluid drive in the converter tends to mask it.

There's several things you can do to diagnose this. 1) A service center can use a scan tool to monitor cylinder misfires detected by the Engine Control Module (ECM). There may even be a diagnostic code stored. Most GM ECM's record cylinder misfire history data which will pinpoint which cylinder has the problem. 2)While the engine is idling, spray a light mist of water from a spray bottle onto the plugs and wires. Look for and listen for arcing and note the engine idle will roughen if a misfire occurs. Often times you can actually see the arc from the wire to ground.

Hope this helps.
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Old 08-12-2007, 05:34 AM   #10
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Great information. This makes a lot of sense to me and will help alot. Provided this is the case, what is the complete remedy? It would seem that this is the opportune time to replace all of the plug wires and perhaps the plugs as well. Is there anything else I should look for the dealer to do? Or is it likely that they would fix the bad wire and leave everything else alone?

I can see where it looks like the snorkle was added to cool the sparkplug wires and prevent the hardening and cracking, but I have no idea if the wires were replaced at that time.

Thanks for your help on this.

gw
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Old 08-12-2007, 06:10 AM   #11
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I'm not sure what the complete remedy would be. It would probably be up to your dealer. There may be a special policy through Workhorse or your coach manufacturer. The burnt spark plug wire issue has been discussed at length on this forum and if you do a search you'll find a lot of information about it. I haven't experienced it with my coach but the general consensus is it is more pronounced on specific manufacturers' coaches depending on the body style.

Generally, spark plugs and wires are considered maintenance items, but I believe these plugs are platinum and don't need to be replaced for 100,000 miles. Personally, if even one wire is burnt I would want all of them replaced. I'm not sure, but I think there may be replacement wires that are supposed to be more heat resistant than the originals.

Hope your problem is small and your dealer gets it all squared away for you.
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Old 08-17-2007, 06:25 PM   #12
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I appreciate all the help on this issue. With your insight I was able to tell my dealer (non-WH shop) that I suspected the problem was a spark plug wire. They expected to find a melted wire laying across a manifold. When they saw I have the 8.1L, the plug wires were not near the manifold and he thought it would have to go to the WH shop for a diagnosis. Taking a second look at it he ran a test on each wire and found one that had pulled loose from metal sheild that connects to the plug. He said they simply reconnected the metal to the wire, crimped it and the engine ran perfectly.

I just drove it home, smooth as silk. It runs quiet and cool. It has great accelleration at all speeds, no shudder and the fans rarely kicked on during the 80 mile trip home.

I can not thank you all enough Dale, DC and especially you Jim for taking so much time to help me understand. It was your help that made it possible for me to educate my dealer. Thanks to this forum I was able to get my coach back in time for the weekend.

I am very excited to have smooth running coach and look forward to putting some miles on it to places other than the dealer!

Thanks again.
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Old 08-18-2007, 01:43 PM   #13
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">It was your help that made it possible for me to educate my dealer. Thanks to this forum I was able to get my coach back in time for the weekend. </div></BLOCKQUOTE>
GREG: THANK YOU for your post. Not too many days ago, you were envisioning all sorts of "major" things that may have failed causing your symptoms. It takes a good and honest person to come back and complete the circle by sharing the resolution of the problem and thanking those who helped you persist with the dealer. Like you, I'm glad it was simply a loose wire. I guess it is just human nature to fear the worst when dealing with any unknown problem. Having knowledgable friends(even the online kind) has saved me lots of times. iRV2.com truly is a marvelous resource, and we are glad you and the others are here. ED
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Old 08-20-2007, 03:01 AM   #14
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That's great news, Greg!

Glad to hear it was a simple fix. Thanks for posting the results.
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