Community Forums

ajm12 · 10-07-2012, 07:57 AM

I have a cat C 7 350 hp. I have the VMSpc installed and it is reporting my turbo boost pressure of around 14 psi. sometimes it drops to 13 and then sometimes it is 15. Reading other post I see some people are getting in the twenties. I was wondering if i might have some kind of problem. If anyone has any insight on this I would appreciate what i should check.

Dr4Film · 10-07-2012, 08:08 AM

Sounds pretty low to me.

I don't own a Cat but I do have the Cummins ISC 350 HP and use the SilverLeaf VMSpc. My top boost has been just over 24 lbs depending on what gear I happen to be in at the time.

What does your dash analog boost gauge display?

Dr4Film ----- Richard

ajm12 · 10-07-2012, 08:13 AM

Quote:

Originally Posted by Dr4Film

Sounds pretty low to me.

I don't own a Cat but I do have the Cummins ISC 350 HP and use the SilverLeaf VMSpc. My top boost has been just over 24 lbs depending on what gear I happen to be in at the time.

What does your dash analog boost gauge display?

Dr4Film ----- Richard

Don't have an analog gauge. I am wondering if possibly my waste gate might be stuck open?

Dr4Film · 10-07-2012, 08:31 AM

If the gate valve was stuck open, you wouldn't get any boost at all. The exhaust brake may need to be lubricated. It is supposed to be done annually. Maybe the valve is only partially closing.

Does the coach "feel" like it has full boost when climbing a grade?

Dr4Film ----- Richard

ajm12 · 10-07-2012, 08:47 AM

I have had problems climbing some hills. Had to down shift some hills to make it up. Any advise on how to check if it is closing or not?

Dr4Film · 10-07-2012, 09:07 AM

Another place to look is your CAC, Charge Air Cooler. If that is leaking at all, the air coming from the turbo will be to hot and the engine will de-rate itself.

I am totally surprised that there is no dash analog gauge for your boost pressure. If you didn't have the SilverLeaf, how would you know how much boost pressure you are getting?

I would think that your normal boost range should be in the low 20's.

My Cummins 350 can be anywhere from the low 20's to as high as 24.4 lbs of boost depending on the grade and what gear that the Allison is in.

Dr4Film ----- Richard

cbeierl · 10-07-2012, 09:16 AM

My 350HP C7 runs at about 25 psi max boost, according to my analog gauge.

ajm12 · 10-07-2012, 09:19 AM

That is a good question. I will have to find out how to check for leaks in the CAC and the exhaust brake.

firedoc · 10-07-2012, 10:31 AM

We have a Cat C7 300hp. It has no analog boost gauge. The Silverleaf shows anywhere up to 27 lbs depending on situation. Btw it's normal to have to downshift to climb some hills to keep it in the power band.

ajm12 · 10-07-2012, 10:46 AM

But does anyone know how to check for leaks in the CAC and the exhaust brake?

rsflight · 10-07-2012, 01:02 PM

A loose or broken clamp can also be a source of a pressure leak. Checking the CAC is usually done by plugging the inlet and outlet and pressurizing it to about 30-40psi and watching the pressure to see if it drops over time, 10-15 min. That will show if you have leaks large enough to require attention. It can be tested without removing it from the vehicle. The exact specs for yours may vary.

Our local truck shop tested ours this summer after a turbo failure sent metal chips into the CAC. Ours tested ok, we were lucky.

Checking operation of either the turbo waste gate of exhaust brake requires unhooking the pivot arm from the attached mechanism. Once disengaged, it should move freely. Annual lube with high temp Never-Seize is suggested.

ajm12 · 10-07-2012, 03:43 PM

Quote:

Originally Posted by rsflight

A loose or broken clamp can also be a source of a pressure leak. Checking the CAC is usually done by plugging the inlet and outlet and pressurizing it to about 30-40psi and watching the pressure to see if it drops over time, 10-15 min. That will show if you have leaks large enough to require attention. It can be tested without removing it from the vehicle. The exact specs for yours may vary.

Our local truck shop tested ours this summer after a turbo failure sent metal chips into the CAC. Ours tested ok, we were lucky.

Checking operation of either the turbo waste gate of exhaust brake requires unhooking the pivot arm from the attached mechanism. Once disengaged, it should move freely. Annual lube with high temp Never-Seize is suggested.

Thanks i will check.

glenn allen · 12-29-2012, 06:17 PM

The exhaust brake has a big problem if it is leaking, maybe like 150 years old and rusted away. The exhaust brake is most likely air operated. It is nothing but a flaper valve that closes the exhaust pipe when engaged. On my 3126/c7 it is operated with a small air cylinder. If it is used on a regular basis it does not need much attention. But if you park the MH and use it only while on vacation then it can become a problem.

What happens is that the shaft that the flapper valve disk is mounted to will rust up in the cast iron housing and stick. It is a simple bell crank operated by the air cylinder, you can move it by hand. You also use graphite oil or powder to lube the shaft. Be careful that you do not use a oil based product that will burn and gum up the shaft. If the shaft is rusted bad enough you may have to use a product like rust buster or WD40. If you do then recheck it to make sure the oil has not gummed things up.

If it sticks closed then you will be unable to make power and will set a ECM code indicating a restricted exhaust. If it sticks open then you will miss the exhaust brake when you turn it on.

The boost on this engine will top out at around the middle 20's. A boost guage is not required. Your side mirror on the tail pipe side will work just fine. If you are making a lot of black smoke on a hard pull then you are lacking combustion air, this condition will show as a lack of boost. About all the average guy can do is check the intercooler for leaks. It is mounted right next to the cooling fan and has large piping going to it from the turbo on one side and on the other side goes to the intake manifold. Leaks here will also cause black smoke as combustion air is missing the cylinders. Next check the air filter and ducting to and from the air filter. If all is OK here then it might be time to have the turbo rebuilt. The 3126/c7 should only make a light smoke under a very hard pull. Almost not enough to see in the mirror. A clean 3126/c7 with good service and correct operator performance does not need a lot of extra guages it takes very good care of itself. Of course if you are one of those who thinks it is ok to leave this engine idleing then you should spend the money so you can watch what you have done to your engine. If you spend a lot of time in the mountains pulling a trailer and loaded to the maximum then a exhaust temp gauge is a fair idea.