I recently installed an Airdog lift pump (Airdog II 4G DF-100) and since then I’ve experienced varying crank times. Prior to the install, the truck would turn over for maybe 0.5 second before firing right up, every time, without fail. And it feels like half a second is generous on the long side. Since the install, crank times vary between the 0.5sec up to 2 seconds. The engine seems to run a bit smoother since the install and no indication of a misfire. The most noticeable impact of the install is varying cranking times. 
 

From what I’ve read the symptoms could be due to worn injectors or poor fuel pressure but would love to hear the collective thoughts of the crew here. I’m thinking maybe 1 or 2 injectors may be going bad. Could crank duration be affected by which piston is up next in the firing order when an injector is failing? I’m sitting at just over 140k miles and I believe I’m on stock injectors and believe in at the end of their life. Injectors are planed for when I do the quad soon. 
 

We did test the pump, which the newer air dog lift pumps are designed to build pressure and shut off until the engine is cranked. I know and have observed the pump cycling when the key is turned to on, just before cranking.  We tested flow when we installed it by observing the return flow line draining into a bucket while the engine is running, which I admit was weak but the documentation also states the original return line will continue to function as before so volume will be split between the two. The install has been in for some time now and no detectable leaks have been observed. 
 

I don’t have a set of gauges yet. My next upgrade is a quad with fuel pressure, trans temp and egt gauges but life keeps getting in the way of the truck budget.

You've provided good information, but I still have a question.  Does lag time in the engine starting happen all of the time - cold engine, warm engine, etc?

 

Fuel injectors could be a problem, but in my opinion - not likely.  I ran my OEM injectors for 303,000 miles and changed them out just because I thought it was time.  They were still performing fine, as in good fuel economy, quick starting, smooth idle, and performance.  

 

Since you noticed the engine starting performance changed after you did the Airdog install, I would be looking for an install-related cause.  One cause that would match the symptoms is small amounts air getting in into the fuel.  Air can be drawn in by the lift pump, or drawn in by gravity when the engine is not running.  So, I would recheck my work.  

 

Slower than normal engine cranking speed can cause a delayed start, as well.  Being that the engine is a compression ignition engine, a slower cranking speed will allow time for each cylinder to leak air around the piston rings, thus extending the cranking time because of less heat building during the compression stroke.   So, if your batteries and / or electrical cable connections are marginal, longer cranking times will occur.

 

The above-mentioned items can be checked without replacing parts and spending money and likely the diagnoses and repair can be performed you.

 

- John

Thanks, John.

 

There’s no discernible change in  cranking rpm. My batteries are good and recently did the WT mod. Terminals might get changed in the next year or two, not because they seem loose or too terribly corroded but because it was previously a light duty farm truck and appear chewed up a bit from jumper cables. I’m a fan of clean hardware. 
 

The variation in cranking doesn’t seem to follow a pattern: cold vs warm, sat for days vs a few hours, etc. It can crank right up in the morning but take a couple seconds to start back up at the gas station. Next time out might be the opposite. 
 

I’ve double checked my grounds and might go over them again. I know for sure the pump consistently cycles when the key turns to on. The new ones are quiet enough you actually have to reach down and put a hand on it to be sure. 
 

It does eventually start every time. It never attempts to fire up and then not. The moment one piston fires it lights right up. Just seems odd. I’d rather know if this is an indication of future problems I can head off early or if it’s a non-issue. I k so there’s a big asterisk here with not having a fuel pressure gauge… the quad might have to come sooner rather than later. 

Too high of fuel pressure can cause hard starts if the L/P regulator is adjusted too high. Are you powering the lift pump off the batteries using a relay triggered and controlled by the ECM? 

 

 

That’s good to know regarding high pressure. 

The new LP harness plugged directly  into the original fuel pump circuit so I imagine the switch to power is controlled by a relay. I would have to look up a diagram or trace the wires back to know for sure though.

I'm not 100% certain, but if you're plugged into the original circuit, I think you're powering the lift pump directly from the ecm. I think you have to wire in a relay so the ecm triggers the relay and the relay sends power to the airdog.

I had to check to be sure - the Airdog harness has a direct connection to the battery with a switched relay controlled by the original pumps harness. 

Let me dive in...

 

Cranking fuel pressure should be 7 to 12 PSI. If it is above 12 PSI it will create a hard start issue.

 

Running fuel pressure should be 14 to 20 PSI. 14 PSI is the minimum to keep the overflow valve open fully and keep return flow going. Above 20 PSI you take a risk at blowing the pump seal out. Yeah I'm sure there is people out there past 20 PSI but I've seen just 22 PSI blow the front seal out.

 

As for an AirDog 100, 150, 165 all use a power relay to supply power and keep the stress off the ECM electrically.

 

Injectors if they have over 100k just replace them. Injectors typically get low pop pressure and get slightly higher idle. Check your engine load at idle warm if close to zero then the injectors are shot.

 

Edited September 23, 20222 yr by Mopar1973Man

Does it start the same if parked uphill? Also cranking speed can slow down gradually and you may not realize it. 

My first Cummins truck was a first gen 1990 WR250.  At one point it became hard to start, long cranking times.   It would start eventually, but had to crank the hell out of it.

 

Diagnosis:  Pinhole in the fuel line. The pinhole was along the frame near the fuel tank. Seriously, it was a tiny freaking pin hole and it was very difficult to find, barely visible to the naked eye.  I finally found it after inspecting the fuel line at night with a bright led flashlight. I noticed small wetspot on a rusty area of the fuel line. Bingo.

 

I replaced that fuel line and truck was back to normal & starting immediately.

 

The slightest amount of air in your fuel system will mess you up.

 

I melted down that old rusty fuel line in my coal forge and made it into a paperweight.

Royal, that’s something I haven’t considered. However,  it’s intermittently  occurred in our flat driveway so seems to be no correlation there. 
 

IronForger, I’ll give that a shot. I’ve checked multiple times for what i’d typically consider a fuel leak - an obvious drip or wet spots at the connections. 

Anything showing even slightly wet will most likely be a leak with dirt/ dust in the area, providing that you wipe everything down nice and dry after install. That way it will show signs or clues of leaking. I always check after a couple runs or even a couple weeks on a new install.

My truck has long cranking issue but I think it is caused by a squashed key on the VP44. 
I tried to buy a new one but the looks from the counter guys & gals told me all I wanted to know. That is for another topic on the soap boxes!

Anyway I dug into my junk found an oversized key filed it down to fit the shaft & gear glued it in the shaft. Fired right up. 
Cold cranks quicker than hot starts. 
Any ideas?

Ironforger tell us about your forge. Been thinking we waste too much tin can iron!
 

I'd be remis if we don't add to this thread and emphasize that the battery terminals must be clean and a bright shiny metal free from corrosion. These trucks are super sensitive to poor connectivity and electrical grounding which can have an effect on the proper running of these rigs. This has been drilled in to me from more experienced Gen 2 owners than me.  

VP44 keys are numbered and calibrated to the pump. There is a article with the part numbers for the different keys which you normally buy from Cummins not a parts store.

When hard starts occur I have heard of a way to determine what;s wrong by pouring a bit of cool water over the PSG on top of the VP44. If it starts right up after cooling then there's an indication the PSG board is going.

 

Everyone, I cannot magnify enough on how important it is to know the source of any rebuilt VP44 before purchasing. If the supplier cannot verify that it was rebuilt by a Bosch certified injection pump rebuilder then stay away from it. Often going for price or going for plenty of sales hype alone will cost you, having to do the job twice etc. You must verify who the rebuilder is and if they are Bosch certified. If the supplier cannot produce verification, then stay away. I buy my VP's direct from the Bosch rebuilder. It costs me a bit more but also insures they'll be in business the next time I come looking for a properly built and fully tested injection pump.

@DrJekyll86, any progress with the long crank time diagnosis?

 

- John

 

@Tractorman I’ve been out of town for work this week and haven’t had a chance to really dig in or check back here too much.
 

I did another scan for leaks before I left and nothing stood out. The quad arrived this week so I’ll be diving into that when I get home and will be able to take a closer look at possible fuel issues/grounds. 
 

I cleaned up the battery terminals when I did the WT mod and give them a wiggle test regularly just to be sure they are tight. Around the same time I cleaned up all the grounds while investigating the abs/emergency break light combo. I need to investigate that further but I believe it’s the harness connection near the back of the cab on the drivers side or farther back. 
 

There’s no indication of a bad battery and when starting the motor turns over at what seems to be an appropriate rate. And there’s no discernible change in starting rpm between starts, regardless of whether ignition is delayed. 
 

So all that to say, no real updates. Hoping I’ll be back home this weekend and able to do a more thorough look.  

Thanks for checking back.  We are always interested in the outcome as to what exactly is the cause of  your symptoms.

 

- John

I’ve spent some time on the truck this week. I meticulously looked over the fuel system and see no indication of a leak. 
 

However, my crank position sensor took a dump on me this week. Luckily I was in the garage when it happened so I wasn’t stuck in a parking lot or on the street somewhere.

 

Is it possible for a failing crank sensor to cause inconsistent crank times? It threw cel twice this week with no noticeable impact to the engine before failing completely today. Engine would crank but not start. I would think if it was impacting crank times it would have thrown a cel the past few weeks as well but not sure? Curious what the experts say on this. 
 

Seems to be fine since replacing the crank sensor.

ECM tends to take it's time before setting a code to prevent false alarms. 

Some faults are hard faults and set codes instantly.