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Engine Discussion of preventative/corrective maintenance and other technical issues regarding your coach's engine.

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  #1  
Old 2 Weeks Ago
dan1000 dan1000 is offline
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Default 86 PT38 8V92TA throttle / turbo issue

Question: Does my throttle cable need adjustment, and if so, what is the right way to go about it?

Background: My 86 PT38 with 8V92TA has a turbo gauge that says it develops only about 12 - 15 lbs of boost under full power and heavy workload. I also find the performance quite anemic up steep hills (no toad).

I've noticed that the first half of the throttle pedal travel has no effect on the engine's RPM, even in neutral. I also noticed that the reason for this is that at idle, the cable attached to the throttle lever on the engine, has some slack in it.

I've also noticed that when the throttle pedal is all the way to the floor (asking for full power), the throttle lever on the engine is not all the way to its stop. With some effort, I can move the throttle lever on the engine further than the throttle cable does. Note that in neutral, putting the throttle to the floor does obtain 2200 RPM. I unfortunately cannot tell you what RPM under heavy load is obtained, since I've only just fixed the tachometer, and haven't taken it out for a run yet.

My diagnosis of this is that there's excess slack in the throttle cable in the engine bay, and that shortening that cable would allow the throttle lever to be moved all the way to full power. At the moment, I don't think I'm ever getting full power. And I think that's why the turbo isn't developing full boost either.

The throttle cable runs from the throttle lever on the 8V92TA, a couple feet forward, then around a curve, to end at the air throttle valve (which is controlled by the pedal). So that is presumably where my focus needs to be for shortening the cable.

Questions:

1) Does this diagnosis sound reasonable?
2) Is there a way of adjusting the throttle cable in the engine bay without actually shortening it?
3) If I have to shorten it, how should I go about that? Do I need to disassemble the air valve that pulls the throttle cable?

Thanks in advance to those who might know.

Dan
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Dan Freedman
Santa Ana, CA
1986 PT38 8V92TA Island Bed
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  #2  
Old 2 Weeks Ago
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Chap Chap is offline
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I would rebuild the CC5 cylinder before I tried anything else. If the throttle is that slow to respond and also fails to fully open you could have seals leaking in the cylinder and or the vent holes could be plugged. Rebuild kits are available.
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Chap and Darra Enfinger
Newton, GA
1982 FC33 (sold)
1986 PT40 8v92
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  #3  
Old 2 Weeks Ago
dan1000 dan1000 is offline
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Hi Chap,

Hmm. The throttle response is not slow, it is instantaneous. I can see the throttle cable moving the instant that the pedal is pushed. Unfortunately, the cable has some slack in it, so that moving the cable doesn't move the throttle lever on the engine until the pedal is about half way through its range of motion.

So I think the solution is either shortening the cable or somehow "repositioning" it at the attachment point to the cylinder, so that there is less slack in it. I believe this would also help the engine reach full power output at full throttle, which it is not currently doing.

Dan
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  #4  
Old 2 Weeks Ago
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badandy badandy is offline
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If there is slack in the cable you need a new cable. You can "adjust" the end of the cable in the cc-5 cylinder by loosening the nut and rotating the cylinder, which is also how you remove it from the cable. My guess is the o-ring on the piston inside is square and the inside bore is gummy. It's a generic round o-ring, nothing special, so you can take it apart without fear. There is also a small vent hole on the cylinder that might be plugged, which would make thing wonky.
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Old 2 Weeks Ago
dan1000 dan1000 is offline
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Thanks badandy. I have a fairly comprehensive O-ring kit in the bus, so may give it a try. I'm guessing I need to "adjust" it by about 1/2 an inch, so not sure whether that's possible or not via rotating the cylinder. Perhaps a new cable is the right answer. I wonder if it is available. I also have the bender cruise control, which may affect the part numbers??
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1986 PT38 8V92TA Island Bed
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  #6  
Old 2 Weeks Ago
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I posted the o-ring ASTM number on here somewhere, I can't remember what it was. It might be that someone had yours apart and just didn't screw the thing far enough together. IIRC there are a lot of threads on the end of the cable.

There are places that can make a new cable, it's not special apparently. I thought it was for a while.
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  #7  
Old 2 Weeks Ago
dan1000 dan1000 is offline
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Default Easy adjustment

Today, I peered around trying to figure out how to adjust my throttle cable. The answer was staring right at me, in the form of a threaded adjustment rod at the point where the cable attaches. to the engine's throttle lever (see photo, but ignore the loose cable in the back of the image, which is the emergency fuel cut-off cable).

I adjusted that as best as I could, and got rid of almost all of the slack. I would have liked to get rid of it all, but there wasn't quite enough thread available. It may be that there is more adjustment on the CC-5 cylinder as per badandy's post. I did loosen the CC-5 nut and had a quick look, but I didn't unscrew the CC-5 from the bus, so could not attempt to thread things on a bit more. But from the engine's throttle arm attachment point, most of the job has already been done.

I started up the bus, and noticed that there is now very little "slack" between pushing the throttle pedal and the RPM changing.

Tomorrow I will take her out for a test drive, and run her under load. I'm hoping my turbo boost now goes up to 23" rather than the 12-15" before.

Thanks to badandy and Chap for your responses. Invaluable!

Dan
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Dan Freedman
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  #8  
Old 1 Week Ago
dan1000 dan1000 is offline
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I took the PT38 out for a test drive today, and found there has been a considerable improvement in power. The previously misadjusted throttle cable was simply not allowing full throttle to be obtained. Now, it seems that it can be obtained, and it is very satisfying when it is obtained

BUT

There may still be one or two issues to resolve:

First issue: When calling for a lot of power (foot to the floor, accelerating or going uphill), the transmission seems to upshift at about 1950 RPM. Since the redline is 2200 RPM, I thought the transmission would upshift at that point, or close to it. 1950 is a long way away from 2200, I think. Note that when I use the transmission panel to hold the transmission in a particular gear (ie: prevent shifting), the full 2200 RPM is reached. I'm wondering whether there is a shift point adjustment on the transmission, or maybe a tie-in with the throttle treadle for throttle position.

Second issue: The turbo boost gauge used to only reach about 12-15 PSI under hard load. Now, with the increased throttle travel, it gets up to about 18 PSI. But I think it should be reaching about 23 PSI.

I'm wondering whether either of these problems sound familiar to anyone, and if so, what the fix might be. Some things I'm contemplating:
- throttle may still not be quite full
- turbo may have an issue

Thanks for any insights

Dan
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  #9  
Old 1 Week Ago
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Boost could be a shoddy old gauge, too.

I think 1950 is about normal for shifting.

My throttle cable end is different than yours, no adjustment there on mine!
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Maybe you have a leak in your intercooler or hoses.

Is the air filter fresh?
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