Reconditioned engine first start, running rough

[coconup]

Originally Posted by ngoma

Agreed that compression test results that show consistent readings balanced across all cylinders is more important that the actual numbers, given that at this stage we can consider the engine not even broken in yet (you did replace the rings and rehone?) and the actual compression should rise during complete break in.

PS I did not know that breaking in is a thing when rehoning and replacing pistons, thanks a lot for the hint. Do you have a rough idea of what to expect from a compression test now, and of how long it should take for figures to improve? I was planning another compression test this weekend with a better set than the chinese one I used so far.

[BogfordGarage]

Are you able to pinpoint when the bubbles appear in the clear fuel line? This might seem like a silly question but when trying to get our 760 running for the first time we had this problem.
It would start okay, throttle up but as soon as you let off the throttle bubbles would appear while idling, then stall. This turned out to be the axial seal on the pump drive shaft, but like you say yours should be fine as it's new. The way we found the leak may help you.
Take a priming bulb attach it directly to the pump inlet and create a vaccum with it, after doing this a hissing noise could be heard and it was traced to the seal. Pressurising the pump didn't reveal anything so it was air getting in rather than fuel getting out that was the problem. I hope this helps.

[coconup]

Originally Posted by BogfordGarage

Take a priming bulb attach it directly to the pump inlet and create a vaccum with it, after doing this a hissing noise could be heard and it was traced to the seal. Pressurising the pump didn't reveal anything so it was air getting in rather than fuel getting out that was the problem. I hope this helps.

Great tip, thank you. I will try this on Saturday and see if I get anywhere. I have the feeling I have a similar problem since I cannot see any fuel leaking. My main suspect at the moment is the transparent hose itself, since I already replaced everything else (except the IP seal of course) without success.

[coconup]

Side note about piston rings break in. I didn't know this was a thing and I hope I didn't screw it up already with all the cranking and misfires 🤦*♂️. Is there a recommended break in oil I should use? Right now the engine is filled with the oil I usually run it with (https://www.autodoc.de/elf/12744089), which is a semi-synthetic 10w-40.

[v8volvo]

Originally Posted by coconup

Side note about piston rings break in. I didn't know this was a thing and I hope I didn't screw it up already with all the cranking and misfires ��*♂️. Is there a recommended break in oil I should use? Right now the engine is filled with the oil I usually run it with (https://www.autodoc.de/elf/12744089), which is a semi-synthetic 10w-40.

I doubt that you are facing any long term trouble with ring break-in. There is a school of thought that says conventional rather than synthetic or semi-syn oil is preferable during the break-in period since it is slightly less slick and (according to this theory) helps rings seat more easily. But there are other folks who dismiss that idea as a fable and say to just run whatever oil in the engine you plan to run long-term. And the latter view IMHO is supported by the recommendations of engine manufacturers themselves for new cars, who almost universally specify synthetics nowadays (at least in any kind of turbo diesel or turbo gas engine) but don't suggest anything about running conventional oils for the first few thousand KM.

As both you and ngoma said earlier, the fact that the compression readings were pretty consistent across all cylinders is a very positive sign, and it's likely you can expect those numbers to rise as the engine runs more, seats the rings, goes through several heat cycles and gets rid of any fuel washdown from starting attempts, and you test with a fully charged battery. If it were me personally I would not even bother testing again until after you have put some miles on the engine. Right now you have already seen results that argue against there being any serious problems, and your readings could be skewed again by the fact that the engine has been hard to start and running poorly. It could be a sort of circular information loop that doesn't actually tell you that much about the underlying soundness of the engine.

A possibly relevant story: ngoma and I working together on a project many years ago fought a massive battle with a D24T that refused to even start. We could not figure out why, after verifying good timing, injection system health, glow plugs, etc. Finally as a last resort, speculating that maybe we had washed all the oil out of the cylinders with extended starting attempts and the rings just couldn't seal anymore, we removed the injectors and filled the cylinders with motor oil (or maybe it was ATF?), rolled it over by hand a few times to wash the oil around in the cylinders, then spun it on the starter for a while to blow the oil back out and prevent hydraulic lock. Reinstalled the injectors and bled the fuel system, and bam, the engine started and ran perfectly on the first try, to my memory. I think it started and ran well ever after too. So it was a lesson in how much the effectiveness of the ring seal against the cylinder bore can be (temporarily!) compromised even in an otherwise strong engine by loss of the lube oil film that provides an important part of the sealing effect.

Look forward to hearing what you find out, this is an interesting and strange issue but it has to be related to the quality of ignition. Normally I would say it sounds just like retarded timing but since you have ruled that out, it does sound like "delayed" ignition despite correct static injection timing, due to poor performing injectors (spray pattern or incorrect opening pressure or delayed actual injection due to air bubbles in fuel) (or low engine compression), could well be the cause.

FWIW, the seat of my pants tells me that a good series of next steps would be to verify that the injectors are working well as you're doing, and resolve any inlet side fuel system air leaks as was advised. But then, assuming you know timing and everything else are correct, try to minimize the amount of time the engine spends running for short moments on the engine stand, and instead install it back into the vehicle, get it filled with coolant, and try to give it a chance to run for extended length of time with a load on it and really get good and warmed up, get the oil hot, etc. That may be what it needs to get all the material washed out of the engine from the internal work that was done and get the rings sealing right.

[coconup]

Thank you for the extensive answer @v8volvo and for the reassuring words on pistons break-in, I was already beating myself up for not doing it right and your post put me in a better mood already .

Next up is figuring out where air is getting in from. Over the weekend I will try the vacuum method with a small pump and see if I can locate the leak anywhere.

[ngoma]

One injector had a bad spray pattern? The rest were OK? Correct POP? What about drips or moisture when held 10 sec. at just under POP?

There are many schools of thought for piston/ring/cylinder break-in, some even adding sink scouring powder to hasten ring seat. I like the "Run it hard out of the gate, under load, varying RPMs, up and down through the RPM range, quick accelerations and decelerations."

http://www.mototuneusa.com/break_in_secrets.htm

Air intrusion:
I have experienced air intrusion from various sources at different times.

• Bad IP mainshaft seal.
  
• Deteriorated/loose injector spill tubes, including the stub (last).
  
• Loose injector spill nipple.
  
• Leaky IP return hose.
  
• Bad IP distribution head large diameter o-ring.

Temporarily installing clear hose can help track down these vacuum leaks. Often there may be stubborn large bubbles in the high spots; these are not important. Watch for a constant stream of tiny champagne bubbles running along the top of the tube.

One of the best methods to determine if air-intrusion is the problem is to temporarily install a low-pressure fuel pump just upstream of the main filter. Run it that way and see if the problem disappears.

Lastly: Air intrusion itself can cause hard starting. If bad enough (usually bad IP mainshaft seal paired with clogged fuel filter), it can cause problems at idle and off-idle.

[v8volvo]

Originally Posted by ngoma

If bad enough (usually bad IP mainshaft seal paired with clogged fuel filter), it can cause problems at idle and off-idle.

A potentially key piece of information here.

[coconup]

Originally Posted by ngoma

One injector had a bad spray pattern? The rest were OK? Correct POP? What about drips or moisture when held 10 sec. at just under POP?

Yes they all pop at the right pressure and only one of them slightly dripped when held at close-to-pop-pressure. Two new injectors are on their way already since they are under warranty, although admittedly I lost a bit of confidence in the seller.

Originally Posted by ngoma

There are many schools of thought for piston/ring/cylinder break-in, some even adding sink scouring powder to hasten ring seat. I like the "Run it hard out of the gate, under load, varying RPMs, up and down through the RPM range, quick accelerations and decelerations."

http://www.mototuneusa.com/break_in_secrets.htm

Great, thanks for the link. I take it from both you and @v8volvo that the oil I'm using should not matter? I can still drain it and replace it with mineral oil, but also not if it won't make a difference.

Originally Posted by ngoma

One of the best methods to determine if air-intrusion is the problem is to temporarily install a low-pressure fuel pump just upstream of the main filter. Run it that way and see if the problem disappears.

Noted, thank you (once again) for the tip.

Originally Posted by ngoma

Lastly: Air intrusion itself can cause hard starting. If bad enough (usually bad IP mainshaft seal paired with clogged fuel filter), it can cause problems at idle and off-idle.

Does this mean that air intrusion alone would not have an impact on idle performance? I currently see quite a lot of air bubbles being sucked in and idle is very rough (the engine jumps around and struggles to keep up when idling). I thought the bubbles would explain it, but are you saying there must be more to it?

[ngoma]

Means that if the IP sucks in enough air it can cause idle, off-idle problems in addition to hard or impossible starting. Actually, enough air in the system will cause problems across all RPM/load spectrum.

For break-in, if I were starting from scratch, I would start w/ non-synthetic oil as a little more insurance for speedy ring seating, esp. since it will be changed out soon. In your case, I would leave the semi-synthetic (not the same as synthetic oil) in it and not worry about that part. Most of the benefit of the break-it-in-hard method derives from just that-- creating higher pressures at the rings.