OM617 injector return fuel flow and rough idle - Page 3 - PeachParts Mercedes-Benz Forum

BillGrissom · #31 11-09-2021, 10:52 AM

Quote:

Originally Posted by Alex E

Guys, could it be the MW injection pump itself? Some kind of cavitation?

Speaking as a mechanical engineer, cavitation is greatly misunderstood. It is liquid vaporizing temporarily due to a low pressure region, such as behind a spinning pump impellor. It quickly condenses back to liquid as the vapor bubbles reach a higher-pressure region (releasing energy in a small point to make noise and even erode metal). Cavitation does not form air bubbles.

There is a related issue where dissolved air can be drawn out of solution by applying a vacuum (or lower pressure region) to the liquid. That can draw air bubbles out of solution. But, there would be no excess dissolved air in the fuel drawn from the fuel tank. If the fuel tank was pressurized to say 1000 psig, it could have appreciable dissolved air, but unless testing liquid rocket engines or working in a chemical plant (me), you wouldn't run into that.

BillGrissom · #32 11-09-2021, 11:02 AM

Quote:

Originally Posted by Theseus

1. Bosch injector pump service manuals have a section on plugging the fuel bypass and all ports. To check for air bubbles, it must be pressurized, then placed in an oil bath. This should show you if there is an opening outside. It is possible, however, that an o-ring could be faulty somewhere on the delivery valve body holder.

Just thinking out loud. Since the fuel supply to the injection pump runs ~12 psig, there shouldn't be any place in the fuel passages within the I.P. that is below atmospheric pressure to suck in air. Indeed, if there were bad seals, you should see fuel dribble out, though it could leak into the IP crankcase (flows to engine oil pan). But, if there was a blockage plus a bad seal, it might suck air on say a piston refill cycle. But the bad seal would also have to act like a check valve, allowing air to get sucked in but not fuel to dribble out. I would think that if the IP was susceptible to letting air into the system, this would be a known problem after 6 decades of Bosch IP use.

People here have long fretted over air in the injection system, but I haven't heard of anyone proving that causes a rough idle. If too much air, the pressure in the injection line will not reach 1950 psig to pop open the injector. Try loosening the nut at the top of each injector, one at a time, and verify a little fuel dribbles out with no puff of gas, and each one should slow and roughen the idle by about the same amount. If that proves OK, I would move on from worrying about the gas in the injection return hose and try other things like verifying the injectors with a pop tester, especially for good spray pattern, and verify injection timing, as both of those can greatly affect idle. If you ever drive out to CA, try the "renewable diesel" you can buy at Propel and Union76 stations. My engines idle much smoother on them, due to the high cetane rating (~75), so don't neglect your fuel quality.

Theseus · #33 11-09-2021, 01:43 PM

Its been on pause but will start back up. Had to get sunroof working and rear window seal replaced. I have the parts to replace seals on oil filter. Leak down test first. If that is fine, I will install injection pump next.

Theseus · #34 07-24-2022, 12:16 PM

UPDATES (search posts on my user name for more detail on related posts):

- I needed to replace my crank seal, so this gave an opportunity to replace more parts in the engine.
- Discovered I had 6 degrees timing chain stretch. 3 degrees after new timing chain. Then added 2 degree offset key (with dial indicator method) to get it within 1 degree of spec. I wish I checked this before getting the injection pump! So please check this first! IF you have and still have rough idle condition, please share this to the group.
- Sourced a very nice 85 CA model injection pump. However, I have regrets because of that emissions sensor attached to it. It requires a very tricky ways to access the top bolt simply to attach the washer and bolt to tighten, but it can be done.
- If you have that pump and don't plan on using that sensor, unbolt the cover and you can unscrew that component from the rack. Reseal it with "right stuff, black". Cut the sensor wire and permanently seal cut cable with RTV.
- I bought the M**********.com pump injection timing kit with drip tube and video. For all the criticism, I have to say that his video and kit was well worth the price.
- I sealed gasket with "permatex aircraft gasket sealer". Won't harden or leak, allows for future adjustments, which were invaluable.
- Got the 1 sec drip timed at 24 degrees and able to repeat it after tightening all the bolts on the injection pump.
- installed a pyrometer on the exhaust manifold

At this point - I have not started the engine. I have removed two potential sources of the issue - the injector pump and the timing. The timing was definitely off spec by 6 degrees.

If this engine runs smooth, I very curious to see if air still comes out of the injector pump return line after acceleration. If it does - I may assume this is normal. Remember, my original injector pump had evidence of corrosion inside.

That's it so far!

carson · #35 07-25-2022, 12:25 PM

Thanks so much for the update! I had been following along this thread for a while because of a similar issue. I had gotten a completely rebuilt pump by Yankee diesel and new injectors, but I'm still seeing air in the return from the pump when revving. It sort of looks like cavitation, but not really. I don't have timing chain stretch, but still do have a bit of a rough shaky idle. Keep us posted on how your efforts pan out.

Theseus · #36 07-25-2022, 12:49 PM

Would you mind telling me if you had the air issue with the original pump and now still have it with the rebuilt one? Did this issue only exist with the rebuilt pump? Any thing else you have done to diagnose it?

carson · #37 07-25-2022, 02:14 PM

I noticed the issue with the old pump as well. It drove me nuts trying to figure out where it was coming from. I directly fed the pump with a bottle to eliminate air ingress from the tank side. I assumed it must be the pump and had both that and the lift pump rebuilt, but I still see the same thing. I can only assume it must be normal.

Theseus · #38 08-05-2022, 06:48 PM

UPDATE:

I'll just post these videos first:

https://youtube.com/shorts/PKh9tCL0L3I?feature=share

As you can see, it starts up like a champ. Running smoother than ever.

Notes: With the new injection pump, I kept the clear lines. Upon hard throttle and stopping, there was a small about of "air" coming out of the pressure release line of the injection pump. However, it was nothing compared to the original video. Therefore, that makes my injection pump, the newer 1985 injection pump that have bubbles that exit the diesel return, with the 1985 having FAR less. Carson said he has bubble during this procedure too. I am concluding that "air" from the injection pump diesel return is NORMAL after a hard rev and then straight to idle, but varies between injection pumps.

Here's what I know worked:

Replace engine shock absorbers - smoothed the rocking

Correcting any areas of air ingress in the lines - smoothed idle a bit

Adjust the dampener bolt - smoothed it slightly.

Replacing the timing chain and correcting timing for the cam AND the injection pump - can't confirm what did better. But, I had to deliberately tweak the injection pump timing beyond the adjustable twist range and got it dialed in within 1 degree using the high pressure method. Chain and camshaft within 1 degree too. Had to manually fix the chain tensioner alignment too with a copper sleeve. It went from rocking/pinging to smoother idle.

Changed injection pump - I can't confirm that was the fix. It improved the pressure relief "air" behavior. Did the injection pump with less "air" result in a better idle? I don't know...

If I were to do everything over again, I would check all the things I did before before swapping the injection pump. The injection pump is that last thing I would check.

carson · #39 08-07-2022, 09:54 AM

Thanks for the update. The video link you shared doesn't want to load for me for some reason. Can you give me a bit more info about the timing chain tensioner shim? I just picked up a new spring to put in mine.

Theseus · #40 08-07-2022, 05:22 PM

#(Fixed the YouTube Link)

Here are two photos of what the chain tensioner looked like when I found it. As you can see, it is misaligned. I had a broken spring, like it was split in two.
In my case, I found a copper tube and made a copper sleeve to fix the misalignment. In order to get a nice tight fit, US diameter tubing will need to be hammered and expanded. In order to make the tubing wall thinner, I hammered the copper sleeve against a steel bar to increase its diameter. Once it slid over the tensioner and chain guide pin with minimal resistance, I greased it, and then aligned it with the tensioner.

Theseus · #41 10-01-2022, 09:39 AM

UPDATE: I found a way to smooth the engine idle even more.

Summary (TLDR): It involves fine tuning of the idle and damper bolt. The objective is to find the optimum idle speed to lower the vibration. Then the optimum rack damper position. Finally, combining both to lower the vibration.

Detailed procedure: Start engine and let it warm up. Back off the rack damper bolt. Then start adjusting the engine idle screw. Start at the low end and work your way up. I found there were two resonant points and a sweet spot of the lowest vibration. The point is to adjust your rpm within the factory spec range - 750ish - until you feel it is as smooth as possible. Mine was around 800 rpm. At that point, slowly add in the rack damper bolt. I found that just a SLIGHT amount was all needed and then the rpm goes up when you engage it more. Now, lower the idle just a little bit and engage the rack damper back up and the idle speed should match the speed prior without the damper bolt. It should feel even smoother with the optimum idle speed and damper bolt. Tighten everything back up afterward.

I found using your body as a "vibration meter" helps when leaning over. You want to dampen that low-frequency wobble. That is what you feel in the car the most. I also had some slight pinging in my number 1 and 2 cylinder - that went away too.

Hope this helps!