Hard cold start mystery 3126

Deville · Jun 13, 2014

So I'm losing my mind a little bit the past 4 weeks. I replaced the IAC valve on the Heui pump and the IAC sensor on the top of the crank case due to the engine always giving me a hard time on a cold start. When it's plugged in it's fine.

Had my mechanic replace the sensor on the crank case and it still gave me a hard time so I ordered a new IAC valve, but I went the cheap way and ordered one of ebay and it came from china.

Install it, started it up cold ran rough for about 20 minutes then cleared up once the system bled itself of air.

However there was an issue, a pretty big issue. The valve I just replaced was leaking oil.

Upon further inspection I found it was leaking from the back end of the valve from a hairline crack in the metal.

So I dealt with this little annoyance for like 2 weeks. I tried using a quick set metal epoxy to prevent it from leaking but it didn't work. However the motor fired up every morning unassisted. After two weeks the entire part crapped out. The sensor just died so there was no signal being sent to the ECM.

I run to a freightliner dealership get a new part, install it...but I did one thing different. Instead of staring it cold I kept it plugged so when I did turn it over it would be warm and the oil would flow easier, the motor would run smoother and less air bleeding. My theory was correct. Motor ran great from the start.

Come in the next morning. Engine won't start. Checked to make sure the sensor was good, it was. Plug it in for 20 minutes and it starts.

I'm so confused. Brand new part from the dealer and it doesn't work?? The only thing I can think of what maybe I damaged it from keeping it plugged in. It's reading a warm condition and not opening or closing properly.I don't know.

I am still getting codes for a bad IAC valve, no check engine light.

Before I run out and drop another $300 on a another new part I want to hear some opinions.

Once it's running it runs fine all day. No other issues to speak of. (knock on wood)

Thanks!

usafk9 · Jun 13, 2014

I know little about Cats, as we've only ever owned Mercedes. I'm spitballing here: cold fuel introduced to warm air....intake heater grid/ glow plugs? I'm guessing if one is cold, other warm, it starts? Both cold, no-go? Like I said, I'm spitballing.

I'm also guessing Davekc or retidepxe will be along soonly.

Sent from my DROID RAZR using EO Forums mobile app

davekc · Jun 13, 2014

Probably need to run a fuel pressure test when warm and on a cold start. Very often the high pressure fuel valve will become cracked because it is plastic. It is located on the back of the block. If it has a hairline crack (which many do over time) it will perform because the block is warm. When cold, it loses pressure and you basically lose your prime. Not because of plastic, but sometimes a internal leak at the lift pump will give you those same symptoms. These issues will not usually throw codes unless the pressure drops at run time. Valve replacement is roughly 250 and lift pump around 400.

Usually on a air intake, it will throw a code if bad at startup. Easy way to test tho. On a cold engine, use a hair dryer on the intake and then see if it starts. If it does, then you have isolated the issue to the intake heater.

Deville · Jun 13, 2014

No issues with the block heater or the grid heater. Like I said if I plug it in for 15-20 minutes it usually starts on the first shot. if I leave it plugged in all the time no issue at all. I keep it plugged in in the winter so I even in the coldest of days that we had this past winter 99% of the time it started right up and according to my scan gauge the temp would be about 72*

I have had the same codes in the truck for years

39 IAC valve
31 speed sensor (had one of the two replaced years ago, not sure which one was replaced)
73 injector fault

When I had put the first new IAC valve in I only had code 31.

I'm beginning to think I may have a wire harness issue.

When I unplug the sensor pluf to the IAC is sounds like it's not getting any fuel. When It's plugged in it sounds like it's dragging and it want's to start it just won't catch.

davekc · Jun 13, 2014

That would confirm the starting place I suggested. It sounds like a low fuel pressure issue when cold. I would do a fuel pressure test and that likely will show your problem. I'm not convinced it is a electrical. If so, it should throw an active code at start up. If the code 31 still shows, might want to replace that sensor. The reason they have two is one takes over if the primary fails. Two bad ones and you have a truck that won't run. Cheap fix compared to a tow bill.

Deville · Jun 13, 2014

davekc said:
That would confirm the starting place I suggested. It sounds like a low fuel pressure issue when cold. I would do a fuel pressure test and that likely will show your problem. I'm not convinced it is a electrical. If so, it should throw an active code at start up. If the code 31 still shows, might want to replace that sensor. The reason they have two is one takes over if the primary fails. Two bad ones and you have a truck that won't run. Cheap fix compared to a tow bill.

Where are those sensors located? I'm told they are PIA to get to.

Also, why would the truck continue to throw a 39 if that isn't the issue?

How would I replace that HPFV? is it accessible from the top of the motor, under? inside the cab?

RETIDEPXE · Jun 15, 2014

If it is a loss of prime when cold, try priming with the hand pump. Could be as simple as hand pump leaking loosing prime when it sits, or as Dave said the HPFV which is probably like my C9 and located at back of head. I am told these fuel pressure valves are notorious for wearing out so in a milliom miles I have replaced mine 3 times just because and it has never left me stranded. Inspecting the old one one can see where the plunger tip shows signs of wear causing it to not seat properly. I can reach mine by going thru the man hole cover on the trans hump inside the cab (automatic trans, a stick shift will present further challenges).

I recently had a no start problem, hot or cold. Fired right up with a spit of ether and ran good after cranking. Turned out to be a $6 top O-ring on #3 injector found by massive amount of oil spraying out of the top of the injector at idle.

I'll pull some CD's of parts I have on the 3126 and see if I can find anything else.

Let us know what you find.

greasytshirt · Jun 17, 2014

When the fuel pump starts to crap out, you'll see metal debris in that valve on the back of the head. It's important to address if you find metal, it'll wipe out the injectors.

Injector o-rings can leak, causing loss of prime. Is there an unexplainable loss of oil? Is the fuel black?

Is the injection control pressure normal?

Trucks with quick-connect fuel lines sometimes have seals in the quick connect couplers leak. They'll have fuel residue on the outside of the line, around the couplers.

Deville · Jun 18, 2014

I replaced the hand pump last year because it wasn't staying closed and sucking air. The prime on the hand pump is good.

When ever I fill up I check the color of the fuel, I can always see to the bottom of the tank at any level of fuel. The antifreeze doesn't smell like diesel and is green.

Truck hardly ever burns oil. I have put 8,000 miles in between oil changes and never had to top it off.
About a year ago I was done about 2 quarts which struck me as odd, I replaced the oil never had an issue after that.

I have very little smoke at start up or at extended idling.

Since I have a manual I can't figure out how to get to that pump, plus i need to see what it looks like.

I also need the location and a picture of those speed senors. Maybe I can so those myself.