Engine/Transmission (1994 - 1998) Loss of boost after head gasket replacement

[JRS]

Had the local Cummins dealer replace my head gasket. But now I've lost about 6 psi of boost, according to my Westach gauge. Before I could pull 30-31psi of max. boost, now maybe 28 psi. If I'm hauling my trailer (10k lbs. ) max. boost drops off to 25 psi. I'm running a TST plate, but it doesn't look like it's been tampered with. Would the head gasket replacement alone cause the drop in boost?

 

[tp0d]

Have you checked your wastgate setting to see if its been changed? I believe cummins wants a max of 28-29psi on the HX35, mebe they reset it. .



Or, you have a boost leak somewhere. Troubleshooting that can be a pain, easiest way is to check your boots for hairline cracks or looseness. . If you can maintain 28psi, its a really small leak. not good.



As for the headgasket change itself causing the leak, prolly not. Its the associated disassembly that caused it.



good luck



-j

 

[JRS]

thanks. I have a rigged up plumber's helper in the garage that I found a leak with before. will pull it down and use it again.

 

[WDimig]

check the boost line fitting, they probably cracked it just enough to make it leak and give you a false number, had that happen on a friends truck, couldnt figure out his boost "leak", there was no leak, just not the right boost signal sent to the gauge, good luck



wes

 

[JRS]

thanks, I'll check it. Unfortunately, it feels like I'm down on power too.

 

[WDimig]

hmmm... . check all the boots on the turbo to intercooler to engine, make sure they are seated right, look, feel and hear for leaks, use some soapy water, spray it on there, if theres escaping air, you'll see it. i doubt they tamped with your waste gate, unless you gave them permission to change the setting, i dont see why they would have touched it.



wes

 

[Matthug]

If he's down on power thats a fuel issue. I'd check your fuel pressure, possibly an overflow valve problem.

 

[JRS]

I had made a tool to pressure up the intake tract two years ago to /look for leaks. I used it again and pressured up to about 5 psi. I used my 2 year old daughter's bubble mix and soaked all the hoses and connections. nothing. I de-pressured it and then pressured it up to 10 psi and soaped it again. nothing. I put a socket on the connections and gently tightened them. (none were left loose from Cummins) Soaped it again, and still nothing. I filled my hand with bubble solution and rubbed it on the hoses and connections. Not one bubble. I crawled under the truck and sprayed/rubbed bubble solution on the intercooler boots and their connnections, still no bubbles. All I can hear is air coming out of my boost elbow. Of course bubble solution on this blows big bubbles. two years ago I had a leak at the metal to metal connection between the base of the turbo and the elbow. That joint is nice and tight and I don't have any leaks.



I changed the overflow valve right before I took it to Cummins for the head gasket replacement. Still had 32 psi of boost with the new valve.



out of ideas as to why a head gasket replacement can cause a loss of boost.

 

[Signal73]

if they went with a thicker headgasket, you could have lost some compression (something like . 5 for every . 10 thickness) maybe that is your lost boost throwing this out there for discussion.

 

[Matthug]

if they went with a thicker headgasket, you could have lost some compression (something like . 5 for every . 10 thickness).

Stock gasket is somewhere around . 060 I'm pretty sure... . the marine gasket which they probably used is . 090''. So if it's . 5:1 for every . 1'', than he only lost not even . 2:1 compression which won't drop boost more than 1-2 PSI.



JRS- Did you check the fuel pressure yet? If you are down on power too thats a fuel issue, I know I has lower boost too with a bad overflow valve.

 

[KingS]

I had a dealer change my head gasket this spring on mine before I me recent head gasket fire ring job.



And I had intial boost loss and power check under hood one of my injector caps was leaking tighten it down power came back a touch about two week later it was back to normal.



Another thing I noticed the dealer did on mine is reuse the gasket between the turbo and manifold and had a small leak never payed to much attention to it. But with my recent head job I reused it again and same leak but allittle more notice able the leak was and more egt's ran high and boost was down all from the little leak put new gasket on and egt's dropped 100 degrees at steady speed and turbo boost 5 psi higher and same throttle positon.



Sorry so long but a couple thing to check out.

 

[JRS]

I'll check all this out. I changed the overflow valve a week before I took it to Cummins. New one from TST. Before I took the truck to Cummins I had 32 psi of boost, now I'm down to maybe 26 psi. It's even worse if I tow my trailer, down to 22-23 psi. Problem is I work in Oil and Gas and we are extremely busy. (I put 60k miles on my truck last year alone) I just don't have a lot of spare time to be working on the truck. That was the reason I took it to Cummins in the first place.

 

[JRS]

I have an old overflow valve, will check the spring and change it out with the new one today. That new Dodge I test drove on Friday is looking better all the time.

 

[JRS]

OK, just "re-adjusted" clamshell clamp and connection at the base of the turbo. (cleaned off o-ring and channel) Now I have even less boost, about 23 psi. Have to get cleaned up for church, will try again this afternoon. I think this is the problem. Plus, most likely, they would have had to disconnect this to get the head off for the new gasket. Funny, still can't get it to make bubbles while pressuring up intake system. perhaps I don't have enough pressure on the system? I can only get 10 psi on it without blowing apart my home made adapter.

 

[WDimig]

does the nut feel like it is tightening up?? if you are still tightening, it is probably just stripping, that will cause that join to open up just enough to leak air, or in your case loose boost. the nut is very soft, and will strip and not evern harm the threads on the stud that it is tightnening down, but i have not been able to get just the nut by itself, so i bought a new clamp. i would get a new clamp from the dealer

part # is 4761274

double check that number but im pretty sure thats the clamp #



wes

 

[WDimig]

by the way you metioned that thy might have to had that off for the head removal, sometimes they dont take off the turbo and exhaust manifold, they leave it hanging there and just take off the manifold botls, much easier to do it that way.



wes

 

[JRS]

Threads are in good shape, but it won't hurt to get a new clamp. I cleaned up everything this afternoon and re-assembled it. (gosh, it was hot and humid) No luck, no gain in boost. (no loss either) I'm going to call Cummins tomorrow and check to see if they turned down the boost, maybe so they could warranty the head gasket repair? Their repair order was very detailed, but I can't find anything about adjusting the boost. The break-off nut I installed when I put in the TST is still intact and the housing is still pushed all the way forward.

 

[Signal73]

Stock gasket is somewhere around . 060 I'm pretty sure... . the marine gasket which they probably used is . 090''. So if it's . 5:1 for every . 1'', than he only lost not even . 2:1 compression which won't drop boost more than 1-2 PSI.



JRS- Did you check the fuel pressure yet? If you are down on power too thats a fuel issue, I know I has lower boost too with a bad overflow valve.

From what I researched before I did mine:

Stock gasket engine would be at 17. 5

. 10 would lower it to 17

. 20 would lower it to 16. 5 compression ratio.



still not sure how this would affect boost overall. It does have an effect on cold starting and overall performance from what i read.

 

[Matthug]

Signal73 you are getting your decimal points mixed up I think. . 20 would be nearly a quarter inch thick! If you mean . 010 give . 5 compression loss that is not even close to true. I do believe . 1'' would give . 5 compression loss though. The marine gasket is . 090'' thick and the stocker is around . 060-. 070 or somewhere in there. So thats only a maximum difference of . 030'', going by a . 10'' = 0. 5:1 loss thats only gonna be like . 17:1 or so compression loss.

 

[Signal73]

10-4 on the decimal points, but I am going to have to hit the search button now!

Joseph Donnelly



Independent Contractor

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Join Date: Oct 1999

Location: Albuquerque NM

Posts: 3,501



If I recall, stock protrusion is about . 025" with the stock gasket thickness. With some hopped-up engines, people have gone to . 020" thicker head gaskets, dropping compression with the 180/215 hp style bowl from 17. 5 to 16. 4 to 1.

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