M1078A1 fuel starting issue

[MatthewWBailey]

I drove 40 miles after an easy start then parked for an hour. Tank was half full. 70F outside. Upon trying to start back up, the engine would kick over briefly then die, tried half dozen times and each time was the same. No codes or check engine light. I thought maybe my tank vent was clogged so I opened the fill cap. Tried cranking again and it was still skipping and dying. Then About 30 seconds of continuous cranking and it began kicking over bit by bit then finally started but was barely rolling over. Then it slowly built up to full idle over about 10 seconds.

Drove 40 miles home and no issues.

I'm wondering if the tank vent could cause this?

Luckily those group 65 AGMs were fully charged.

 

[GeneralDisorder]

Have the fuel lines been changed? If not that is the likely culprit. Sucking air into the system. Every one of these trucks has bad fuel lines. It wasn't a great choice of hose for that application. Definitely biodegradable though!

Or..... the HEUI fairy cometh!

 

[Ronmar]

Primer primer primer! Won’t start, push the primer. How does it feel? If you pump it rapidly, you can build 10 PSI of primary pressure with the primer. If it is properly maintaining prime, you should be immediately able to feel the resistance caused by the regulator when you press the primer button. If it is not immediately providing that resistance, you are probably compressing air…

An air leak on the primary system sticks out like a sore thumb on a primary pressure gauge…

 

[MatthewWBailey]

Have the fuel lines been changed? If not that is the likely culprit. Sucking air into the system. Every one of these trucks has bad fuel lines. It wasn't a great choice of hose for that application. Definitely biodegradable though!

Or..... the HEUI fairy cometh!

Hoses are all new last August

 

[MatthewWBailey]

Primer primer primer! Won’t start, push the primer. How does it feel? If you pump it rapidly, you can build 10 PSI of primary pressure with the primer. If it is properly maintaining prime, you should be immediately able to feel the resistance caused by the regulator when you press the primer button. If it is not immediately providing that resistance, you are probably compressing air…

An air leak on the primary system sticks out like a sore thumb on a primary pressure gauge…

I tried it again this AM. I pushed on the primer button, it was loose on the first push and tight on the second. But Same starting symptoms exactly, long periods of skipping and barely turning over, then eventually it starts up slowly. I let it run 10 minutes then shut it off. Then immediately restarted it with no issues whatsoever.

 

[GeneralDisorder]

Get into CAT ET and run a HEUI pressure diagnostic. You can increase and decrease the pressure manually. You are looking at stability of actual pressure vs the target pressure. Should be +/- 5 psi or so. If it's jumping around a bunch it could indicate a problem and it needs to get the pressure up above 870 psi to start.

 

[MatthewWBailey]

Get into CAT ET and run a HEUI pressure diagnostic. You can increase and decrease the pressure manually. You are looking at stability of actual pressure vs the target pressure. Should be +/- 5 psi or so. If it's jumping around a bunch it could indicate a problem and it needs to get the pressure up above 870 psi to start.

Ok I did the test. It's running 880-862 when the set point is 870 but settles to 873-867 over a few seconds . 18% output command. Steps up and down normally. Only code or flag is...

 

[MatthewWBailey]

Also got this...

 

[GeneralDisorder]

Well that looks normal. Excellent.

So how is the Ether bottle and the grid heater?

You need the Atmo sensor for the 330 tune. But it's a silent code - won't show a check engine light. It is CAT #194-6722. It plugs into the harness - there's already a port and connector for it at the top of the engine.

 

[GeneralDisorder]

Engine speed no pattern..... well that's not one I have seen.....

Code description and fault condition seem to suggest "intermittant or complete loss of signal from the primary speed/timing sensor for 2 seconds"

I would be checking the timing sensors and their wiring on the drivers side rear of the timing case. Those are FUN to get to! I had to unplug and replug those to change my main engine harness. The wiring on these engines is really suffering from age. Had to change all of mine out due to the damaged injector harness and CAT changing the bulkhead connector on the rocker cover riser so that meant changing the whole engine harness to match. Glad I did it. Wasn't thrilled with the $2k price tag on the parts. But there's not substitute for NEW engine wiring if you can still buy one. Buy once, cry twice or thrice.

 

[Ronmar]

I tried it again this AM. I pushed on the primer button, it was loose on the first push and tight on the second. But Same starting symptoms exactly, long periods of skipping and barely turning over, then eventually it starts up slowly. I let it run 10 minutes then shut it off. Then immediately restarted it with no issues whatsoever.

OK, sounds like it mostly held its prime, now you need to hookup a primary fuel pressure gauge to insure the lift pump is doing its part. Should have ~10PSI when cranking, ~25 at idle, ramping up to 60 by around 1200 RPM… You could have a perfect prime but if the lift pump isn’t pressurizing the gallery, the injectors are not going to fill and deliver fuel properly…

your symptoms sound like it has a fuel feed issue, Barely start, stumble stumble stumble, run normally… Immediate Restarts just fine… but it of course could be something else.

most EFI will simply not function at all without the timing signals So I can see it not wanting to catch if it was missing timing while cranking. But that is also the type issue that if it did allow start would start right up normally, but could also simply shutoff if that signal drops while driving…

They often can still operate at a general preset “limp home” value when missing other pressure or temp sensors…

 

[MatthewWBailey]

Well that looks normal. Excellent.

So how is the Ether bottle and the grid heater?

You need the Atmo sensor for the 330 tune. But it's a silent code - won't show a check engine light. It is CAT #194-6722. It plugs into the harness - there's already a port and connector for it at the top of the engine.

The atmos sensor is on there. Grid heater pulls 88 amps. Ether bottle is empty and the black lines are all busted off. I've not fixed them yet.

 

[MatthewWBailey]

Engine speed no pattern..... well that's not one I have seen.....

Code description and fault condition seem to suggest "intermittant or complete loss of signal from the primary speed/timing sensor for 2 seconds"

I would be checking the timing sensors and their wiring on the drivers side rear of the timing case. Those are FUN to get to! I had to unplug and replug those to change my main engine harness. The wiring on these engines is really suffering from age. Had to change all of mine out due to the damaged injector harness and CAT changing the bulkhead connector on the rocker cover riser so that meant changing the whole engine harness to match. Glad I did it. Wasn't thrilled with the $2k price tag on the parts. But there's not substitute for NEW engine wiring if you can still buy one. Buy once, cry twice or thrice.

Are those behind the Air compressor?
These..?

 

[MatthewWBailey]

OK, sounds like it mostly held its prime, now you need to hookup a primary fuel pressure gauge to insure the lift pump is doing its part. Should have ~10PSI when cranking, ~25 at idle, ramping up to 60 by around 1200 RPM… You could have a perfect prime but if the lift pump isn’t pressurizing the gallery, the injectors are not going to fill and deliver fuel properly…

your symptoms sound like it has a fuel feed issue, Barely start, stumble stumble stumble, run normally… Immediate Restarts just fine… but it of course could be something else.

most EFI will simply not function at all without the timing signals So I can see it not wanting to catch if it was missing timing while cranking. But that is also the type issue that if it did allow start would start right up normally, but could also simply shutoff if that signal drops while driving…

They often can still operate at a general preset “limp home” value when missing other pressure or temp sensors…

I'll have to conjure up some fittings to test that fuel line pressure. Is the "primary" pressure coming from the tank feed line?

 

[Ronmar]

Primary pressure is created by the fuel lift pump in the HEUI pump. There should be a T fitting on the hard line between the HEUi pump and the main fuel filter. Mine was an 1/8NPT Port with a pipe plug screwed in.

You can use a simple mechanical oil pressure gauge but you need a needle valve inline or squeeze the poly line that comes with most mechanical oil pressure gauge kits, to dampen the needle pulsations as most of these gauges do not have snubbers installed. On An un-dampened gauge the needle will sweep between 10 and 80 PSI in a blur with each lift pump stroke and be unusable. You want to squeeze the poly line against a frame with a c-clamp or vis-grip, or close down a needle valve just enough to stop the pulsations but still allow the needle to track to, and read the average pressure… i show a cheap oil pressure gauge setup in one of my fuel system videos pumping to 10 PSI with the primer…

here is a video of what an air leak looks like on a dampened/snubbed primary fuel gauge… you can have some pretty unstable primary pressure before the engine ever shows running issues, but if everything is sealed that pressure should be steady unless changing RPM…

 

[MatthewWBailey]

Primary pressure is created by the fuel lift pump in the HEUI pump. There should be a T fitting on the hard line between the HEUi pump and the main fuel filter. Mine was an 1/8NPT Port with a pipe plug screwed in.

You can use a simple mechanical oil pressure gauge but you need a needle valve inline or squeeze the poly line that comes with most mechanical oil pressure gauge kits, to dampen the needle pulsations as most of these gauges do not have snubbers installed. On An un-dampened gauge the needle will sweep between 10 and 80 PSI in a blur with each lift pump stroke and be unusable. You want to squeeze the poly line against a frame with a c-clamp or vis-grip, or close down a needle valve just enough to stop the pulsations but still allow the needle to track to, and read the average pressure… i show a cheap oil pressure gauge setup in one of my fuel system videos pumping to 10 PSI with the primer…

here is a video of what an air leak looks like on a dampened/snubbed primary fuel gauge… you can have some pretty unstable primary pressure before the engine ever shows running issues, but if everything is sealed that pressure should be steady unless changing RPM…

Which one do you mean...
The line from the fuel filter is a hose

 

[Ronmar]

It should be farther to the rear. the test port should be on the line between heui lift pump and secondary filter(metal pipe/line). They usually have test ports at the input and sometimes at the output of the secondary filter, which is near the back of the cylinder head In the LMtV install…

Fuel flow on a 3126 is:
tank
primer/filter
heui pump(lift pump)
secondary filter
front of cylinder head
thru head gallery past injectors
pressure regulator on rear of cylinder head
return to tank.

 

[hike]

On the 2003 3126b M1078A1 the tee is rear of the primary just under the structure carrying the rear cab lock—

 

[GeneralDisorder]

There is a primary and secondary timing sensor and it is likely that the ECM can run in a reduced state with just one of them. That is common in the EFI world. Fallback strategy.....

Likely a wiring/connection issue I should think but sensors are also possible. That code is VERY SUSPECT for the cold start issues. I would immediately stop and correct that before proceeding.

 

[GeneralDisorder]

The atmos sensor is on there. Grid heater pulls 88 amps. Ether bottle is empty and the black lines are all busted off. I've not fixed them yet.

This is just ALL pointing to that timing sensor code. The ECM has to "sync" to the timing sensors every time you restart the engine. If the signal isn't stable it will take a bit for the ECM to either see enough of the uninterrupted signal OR for it to give up and switch to a fallback strategy. So the rough delayed start is PROBABLY the timing sensors, wiring, or (woof) the trigger teeth on the timing gears.