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Discussion Starter #1
1999 Suburban 205,000.



I purchased and installed a rebuilt IP in December from local Stanadyne shop, as pump was shot. I then had same shop set timing after i installed it. Shop had some problems getting it timed but then replaced the cam sensor and all was good.

In February/March I started getting p0251 code, would clear and truck would run fine. Two weeks ago code appears again, I found leak in FFM lower small seal so I replaced it, fuel filter and all rubber fuel lines, cleared code and it ran great.



Tuesday wife back out of garage and it stalls, I have her run scan and its p0251. As I was out of town for work I had it towed to Stanadyne shop. Mechanic called me yesterday and stays he has been running it and the engine will "miss" randomly, doesn't matter if hot or cold. he also tells me the timing has moved half a degree sense he set it back in December so he believes the timing chain has stretched as it has 205,000 although he has not dug into the engine as it was add to labor costs. I asked about the optical sensor and he just stated it was built into the pump so.....I mentioned I changed the one on the old pump at 125k and he just repeated its part of the pump. I had to leave it at shop until Monday as I was not near the shop and it was closing time.


I am not dismissing the chain could be stretched but I would like to try other options before tearing the engine down. From reading forums I am going to do the follow:
1) replace PMD with my spare
2) test lift pump (original)by turning water drain valve on near thermo housing while running
3) hook clear line at water drain and see if air in fuel
4) replace optical sensor.



Mechanic seemed convinced it was mechanical as timing has moved, my question is would any of the above cause timing to move when it "misses" ? Wife stated it would jerk randomly before this happened... I rarely drive this vehicle so I never knew about this.
 

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The PO251 code is almost the death code for the IP. >>
Cause
Engine running, then the PCM detected eight (8) consecutive Camshaft signals were missing, or that an average of eight (8) Camshaft signals were missing for 32 No 1 Cylinder events.


So your encoder (Optic Sensor) is not picking out signals from the disc in the IP. So what do you do? First make sure there is no "Electronic Filter" plugged into the encoder. Next make sure no air or dirty fuel is entering the IP. Third make sure they Installed a new encoder in the IP.

Fix might be a new encoder or a new IP


And your timing chain should be replaced around 150K for best performance.
 

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IP bad after only 5 months...?


Unlikely. I would check if the optical filter is still present and promptly remove it.
 

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The encoder is quite expensive so rebuilders often reuse this part.
:ditto:

Replace the roller chain set. The melling brand I tried recently is very nice. I like it alot more than the cloyes. Eeek 205k...

While your at it replace the front main seal (crankshaft seal) and thoroughly inspect the harmonic balancer and crank pulley. :rolleyes: bad balancer can cause random missing... along with alot of other random bad things to happen... random bad things you DONT want to happen.

The rubber insert should be uniformly flush on the back side of the balancer. No missing chunks, no cracking (of the rubber) same goes for the crank pulley... theres a large rubber insert in the center. Check it for cracking and missing chunks. Also check to make sure its not completely dislodged. It shouldn't budge at all. With the engine off and belt removed, the crank pulley should have no wiggle, no play. Very firm. Crank pulley bolts are 20ft lbs, hb center bolt is 200ft lbs.

Under the timing cover;
Ip gear (3 bolts) 20ft lbs
Cam bolt 65/8?-120 something. Depends on year. Check stickies.

Removing timing cover;
Leave CPS in cover. Just unhook.
Remove power steering pump, but leave lines hooked to it. Use a few zip ties and hang it off to the side. Rest is pretty self explanatory I'd think....

:thumb:

... torque specs...

https://www.dieselplace.com/forum/3036559-post2.html
 

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150K timing chain? Really? Wow.

I guess I better pull the front off all my trucks and replace them... I'm at389K on my burb, 265 on my Sierra, unknown miles on the engine in my 96 K2500, engine is out of my 2000 C2500... no codes, they run well... my Sierra does randomly fishbite so, it couldn't hurt I guess...
 

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Discussion Starter #9 (Edited)
thank you for the replies.


:ditto:

Replace the roller chain set. The melling brand I tried recently is very nice. I like it alot more than the cloyes. Eeek 205k...

While your at it replace the front main seal (crankshaft seal) and thoroughly inspect the harmonic balancer and crank pulley. :rolleyes: bad balancer can cause random missing... along with alot of other random bad things to happen... random bad things you DONT want to happen.

The harmonic balancer is original as far as I know (purchased at 102k) I think that is order now after reading this. I am not inclined to change the timing chain right now as I feel its early and the mechanic is only guessing as he did not take anything apart. Additionally, my spare vehicle was just in an accident so i need this one up as soon as possible.



So I am thinking doing the following:


Change pmd with spare

HB and pulley
remove optical filter if it has one (don't remember seeing one when I replaced pump)



if this does not do the trick I will replace optical sensor


thoughts ?
 

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I agree with what DieselPro said. I had this code and iirc it was EITHER crappy, cloudy fuel or a lack of fuel supply. I can't remember now.
 
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Add fuel filter to that list and make sure there is no leak between the filter and IP.
 

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150K timing chain? Really? Wow.
...
Quote the whole thing >>>And your timing chain should be replaced around 150K for best performance. <<<

You will notice I said best performance. While the chain will work fine for years and years it does get slack in it over time. From dirty oil, wear, or whatever. The real problem is the injection pump is tied into this system.

The injection pump does not like to "chatter". Basically vibrating back and forth with pulsations. The fuel output can do weird things when this slop appears. I have seen it on a test stand do all kinds of things when the drive coupling is loose. Most test stands built today have 50 lb. flywheels in them to eliminate this chatter. So like I said 150K for best performance. Just might restore the like new truck performance in an old truck.
 

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Discussion Starter #14
So I picked up my suburban today and drove it home. Talked to mechanic, he stated he cleared codes and spent less than 15 minutes on it. He re-learned everything with the cpu, checked TDC and it is at -.92, TDC was at -.58 back in December so he believes this is due to chain slack. I asked about optical sensor and he said I am welcome to remove the pump and bring it in so they can test it, but he strongly feels its the chain.



Can someone who has more knowledge than me tell me if the chain slack is the only reason for the timing to change? could a bad optical sensor or worn harmonic balancer cause this ?



Suburban ran perfect home, but I know the P0251 code is soon to show up.
 

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How is the truck running now?
 

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Discussion Starter #18
Truck ran great on the way home. About 30 minutes from shop to home, no codes or issues. I have it parked for now until I return home on Saturday to change out HB and pulley as well as pmd and look closer to see if there may be a optical filter present.
 

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I know it is a Preventative Maintenance but if there nothing wrong with the HB and Pulley, just leave it alone.
 

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Discussion Starter #20
Even if it’s never been changed? So you are saying it’s not likely causing my problem.
 
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