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  #1  
Old 02-12-2009, 03:50 PM
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IP bypass valve service

I know there are already some posts here about the fuel bypass valve but I searched for a long time and didn't find much so I thought I would post about this part specifically in case someone else is searching. I'll spell out my lesson and maybe others have stuff to add.

For the past couple months my '84 300DT was exhibiting symptoms of fuel starvation (arrhythmic shaking, hesitation, dramatic loss of power, etc.). I went over fuel supply parts several times convinced that there must be something leaking or constricting flow. I replaced all the filters, all the hoses, all the hose clamps and the primer pump a couple times, checked the lift pump, tightened everything to the point of insanity. The problem only got worse. Symptoms were especially noticeable at times of high fuel-flow, eg: up a hill, accelerating from a stop, passing, etc. It got so bad the car would barely get up to 15mph but unlike a real fuel starvation scenario it never completely stalled and it always started fine.

Finally the culprit turned out to be the fuel bypass valve. The valve is inside of the banjo fitting on the fuel return hose that leaves the back of the injector pump and heads back towards the fuel filter. It is easy to remove, just pull the banjo bolt and separate the two pieces. Inside is a spring and a ball bearing; the spring pushes the ball bearing into a seat inside the bolt. The purpose is to gently resist the flow of fuel _out_ of the IP, thereby making sure the IP gallery is always full of fuel. My spring had worn and the ball was moving up inside of the spring rather than pressing against it. The result was that fuel was leaving the IP too quickly and causing starvation-like symptoms when the IP needed lots of fuel.

To repair it, all I had to do was flip the spring over because only one end was worn. I have heard of other people stretching the spring with their fingers to encourage it to put more pressure on the ball. The best fix is to replace the spring with a new one, 26-27mm long.

Seems like the bypass valve is a wear part and will give out on everyone sooner or later. I encourage you to check it if you have fuel flow problems. It's an easy check, and an easy fix if it's worn.

Aloha,

-Kanoa

Good luck and

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Old 02-12-2009, 04:08 PM
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Image

Here's a drawing of it.
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IP bypass valve service-screenshot-7.png  
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  #3  
Old 02-12-2009, 04:58 PM
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Very interesting and easy to check if one has the symptoms you describe, nice detective work.
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Old 02-12-2009, 09:14 PM
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Is this the pressure regulator spring that everybody talked about stretching to boost internal IP fuel pressure? There was a rather long thread about it recently - if this is the same thing...
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  #5  
Old 02-12-2009, 11:31 PM
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Quote:
Originally Posted by rcounts View Post
Is this the pressure regulator spring that everybody talked about stretching to boost internal IP fuel pressure? There was a rather long thread about it recently - if this is the same thing...
Yes, the thread that Cervan started.
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Old 02-12-2009, 11:53 PM
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Rack Dampner Pin

Here is CERVAN`s Rack Dampener Pin thread'

www.peachparts.com/shopforum/showthread.php?t=234609

KANOA, good dective work.

Charlie
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  #7  
Old 01-18-2010, 06:02 PM
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Very helpful thread

The thread titled "won't start for anything" was very helpful in getting me to isolate my problem. After researching more posts I came across this one and knew this was my problem. My engine seemed fuel starved. I went through the troubleshooting list but all along it seemed I had a fuel delivery problem. When I set up separate intake and exhaust fuel cannisters, and saw fuel coming out of my cigar hose into the exhaust fuel cannister yet when I cracked a nut of a fuel line going into an injector and no fuel came out when I turned over the engine, I knew the IP was pumping but for some strange reason no fuel was pumping through my metal injector lines.

When I removed the exterior bolt on the IP I found a worn spring inside the fuel return bypass valve. Attached is a picture of the spring. It is supposed to be 27mm but as you can see, it has depressed to 20mm hence there was no pressure on the small metal bearing against which the spring pressed. This allowed all the fuel to bypass the Injector and flow straight to the return line and into the gas tank. For those of you researching and reading this thread, my engine is a 617, early 80's turbodiesel.


Autozen's comments and troubleshooting outline in the "won't start for anything" were most helpful. I feel it is cheating to ask before you bust your tail and research. BTW, nothing like spending 20 bucks and figuring out how to hook up a button from the battery to the starter so you can turn the engine over without climbing into the car.
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  #8  
Old 01-18-2010, 06:08 PM
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Roy (whunter) has the last 2 replacement springs in the USA. He was trying to source replacements locally.
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  #9  
Old 01-18-2010, 08:24 PM
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Yes

Quote:
Originally Posted by layback40 View Post
Roy (whunter) has the last 2 replacement springs in the USA. He was trying to source replacements locally.
I am checking spring manufacturers current production...


Here is another thread on topic.

Fuel injection pump starvation with good a good lift pump
Fuel injection pump starvation with a good lift pump






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Last edited by whunter; 01-18-2010 at 08:36 PM.
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  #10  
Old 01-18-2010, 08:59 PM
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great thread,i had a cummins 12 valve that had a similiar setup.i wonder if the springs are the same.........
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  #11  
Old 01-18-2010, 09:35 PM
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Does stretching out the spring work?

Does stretching back the spring back to its original length (27mm) work in a 617, turbodiesel engine? or is the replacement of the IP bypass valve required? I have read other posts and there seems to be a few different views on the matter. I personally think if in doubt, replacing the spring would be more than enough to complete the repair. I also wonder whether I should post this question to this thread or the other that whunter directed me
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  #12  
Old 01-19-2010, 06:56 AM
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Does stretching out the spring work?

yes.
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  #13  
Old 01-19-2010, 08:01 AM
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Quote:
Originally Posted by Bob300DSD View Post
Does stretching back the spring back to its original length (27mm) work in a 617, turbodiesel engine? or is the replacement of the IP bypass valve required? I have read other posts and there seems to be a few different views on the matter. I personally think if in doubt, replacing the spring would be more than enough to complete the repair. I also wonder whether I should post this question to this thread or the other that whunter directed me
More important is what pressure is present in the base circuit of the injection pump. All springs in the relief valve were not the same length in production . Some around 20mm and some around 27mm. Fifteen to nineteen pounds pressure in the injection pump is in the sensible ballpark. Favor nineteen if possible. Many things like the lift pump being tired or partially obstructed filter etc can drop this pressure as well remember. Low pressure will not always be the relief valve. In most cases you do not even know it is low unless as severe as the posters was.

You want this for many reasons. The car will run seemingly not too bad usually with less pressure.

If it is a 240d with the 616 engine it may be even more critical to have somewhere near the right pressure in my opinion. Do read the extensive thread posted on this thread about the relief valve and pump pressures. For too many years this area was ignored.
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  #14  
Old 01-19-2010, 08:06 AM
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Quote:
Originally Posted by barry123400 View Post
Fifteen to nineteen pounds pressure in the injection pump is in the sensible ballpark.
too low. needs to be 30-35psi.
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  #15  
Old 01-19-2010, 02:11 PM
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For my 84 300D the Factory Spec for the Overflow/By Pass Valve is:
Opening pressure in bar gauge pressure
idle 0.6-0.8 minimum
at 3000 rpm 1.3 minimum

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