Hard to start when at operating temperature

[Rohls]

:banana: :banana:

 

Okay, didn't find anything that addressed my current issue whilst performing a little search, so here goes.

 

My 1992 MJ 4.0, 5-speed with ~80K on it is great to start when cold: it takes like one good revolution of the motor and idles great....problem it is hard to start when at operating temperature....like when I go to start it after 20 minutes at the grocery store.

 

Couple things I thought it was related to that I fixed recently:

 

1. CPS replaced (slightly improved idle)

2. IAC replaced (no change)

3. New battery (was going bad anyway....slightly improved start...not really though)

 

One thing I keep thinking about is the fuel pressure...

 

I have a gauge at the rail (thanks Hornbro!)...IDLE= 41 PSI, anything greater than idle is 49 PSI. I have larger injectors and the appropriate FPR...the question is this: is there supposed to be fuel pressure just before the engine turns over (key ON)? and just after the engine is shut down...? My situation is that the pressure goes to 0 PSI right after the motor is shut down and when the key is ON and I hear the fuel pump for a second....I heard there was a valve in the fuel pump that could go bad?

 

Any help would be great.

 

Thanks! :cheers:

[88whitemanche]

What is your cps output?

[Dillithium]

Do you start your car immediately, or do you give the fuel pump time to prime?

Problem with hot starting could point to your fuel rail being heat soaked, but that is hard to verify.

 

How does your fuel return line look? Could it be leaking?

[88mjblue]

Leaky fuel injector(s)?

[Rohls]

What is your cps output?

Not sure; how to check quickly? It's a brand new Mopar part, so I would like to think it's solid and functioning well 500 miles in...thank you!

[Rohls]

Leaky fuel injector(s)?

This could be! The company I bought the rebuilt bosch 703 (mustang 5.0L) injectors from said they have had 'issues' with these and sent me a set of 22 lb/hr Siemens injectors as a replacement...these are supposed to be more reliable...thanks for the tip!

 

Would this explain my 'low-to-no fuel pressure' situation described above?

[Rohls]

Do you start your car immediately, or do you give the fuel pump time to prime?

Problem with hot starting could point to your fuel rail being heat soaked, but that is hard to verify.

 

How does your fuel return line look? Could it be leaking?

When the motor is cold, I definitely let the pump prime (key ON for ~3-5 seconds) and that seems to help...though I don't believe I register pressure on the rail, which is interesting...maybe I need to double check.

 

I'll check the fuel return line today, but haven't noticed it wet/leaking/dripping before.

[Rohls]

So what is the proper fuel pressure protocol....? In terms of Key ON and Key OFF? Should I expect pressure at the rail with the key ON (motor off)? If so, what value? 10 PSI? 44 PSI?

 

Thanks!

[HOrnbrod]

Yes, there is a check valve at the outlet end of the fuel pump. According to the FSM, The system should retain down to 50% of the running engine fuel pressure (~15 psi) for 30 minutes after engine shutdown. A fix for a fuel pump leaky check valve w/o replacing the pump is to install an inline check valve in the supply line as close to the tank as possible. My 91 will hold pressure for only 10 minutes or so, but I do not have a hard starting problem in any conditions. I do have an inline check valve though ready to install in case it won't hold pressure anymore after shutdown.

 

Another cause of fuel pressure loss (besides obvious leaks of course) is fuel leaking past the regulator on the fuel rail. The FSM does not show any tests to check the regulator. The only cure is to replace the regulator.

[optimus2]

coil on its way out?

 

an xj I used to run had this problem, would start and run fine till it was hot, then a hard start and would not run good till it seemed to clear itself out and burn off extra fuel. 

 

coil was getting heat soaked. and was not making the contact it needed to, so it was a weak spark/no spark.    new coil was instant fix.    Not sure this is the case for you, but might be worth considering.

[Rohls]

coil on its way out?

 

an xj I used to run had this problem, would start and run fine till it was hot, then a hard start and would not run good till it seemed to clear itself out and burn off extra fuel. 

 

coil was getting heat soaked. and was not making the contact it needed to, so it was a weak spark/no spark.    new coil was instant fix.    Not sure this is the case for you, but might be worth considering.

Thanks! I replaced the coil last month...the OEM one had a small crack in it.  The new one didn't seem to help much....:/

[Rohls]

Yes, there is a check valve at the outlet end of the fuel pump. According to the FSM, The system should retain down to 50% of the running engine fuel pressure (~15 psi) for 30 minutes after engine shutdown. A fix for a fuel pump leaky check valve w/o replacing the pump is to install an inline check valve in the supply line as close to the tank as possible. My 91 will hold pressure for only 10 minutes or so, but I do not have a hard starting problem in any conditions. I do have an inline check valve though ready to install in case it won't hold pressure anymore after shutdown.

 

Another cause of fuel pressure loss (besides obvious leaks of course) is fuel leaking past the regulator on the fuel rail. The FSM does not show any tests to check the regulator. The only cure is to replace the regulator.

Thanks Hornbrod!

 

My system pressure definitely is NOT what you described...:/

 

Where do I get the proper replacement check valve? Can I install an inline check valve now (without messing with the fuel pump) as a precaution? If so, where can I get one of these?

 

Thanks again...!

[HOrnbrod]

From NAXJA. This one or similar 3/8" fuel check valve:

 

The BMW fuel check valve part number 16 14 9 068 988 fits between the filter and the fuel rail since its connections
match this section of fuel hose. I installed one a couple days ago and it works fine. The unit comes with about a foot of 8mmx13mm fuel line, hose clamps, insulation and plastic nut covers for $3-5. It is necessary to make room for the valve, but that is not hard. Call a dealer and order one, or it may be in stock.

 

http://www.bmwe34.net/E34main/Trouble/535ihot_start.htm

 

http://www.naxja.org/forum/showthread.php?t=1063655

[Rohls]

Thanks all.

 

i check and there is no leaking form the fuel lines. Good news.

 

i plan on replacing the injectors soon so i may rule them out.

 

I am hesitant to put the BMW inline fuel check valve in...I've read bad things about higher pressures it seems...and since I'm running the 49 PSI regulator would this issue be exacerbated?

 

The FPR is newer and I pulled the vacuum off it with no leak...is this a good enough 'check'? How else do i tell if this is the culprit short of spending $50 on a new one...?

[HOrnbrod]

The BMWs run a higher FP than you are running. This won't work on the 97 and up XJs because they have no fuel return line. On your truck it's no problem.

 

Did all this hard starting when warm start with your new injectors / regulator?

 

What do your plugs look like? Running rich?

[Rohls]

The BMWs run a higher FP than you are running. This won't work on the 97 and up XJs because they have no fuel return line. On your truck it's no problem.

 

Did all this hard starting when warm start with your new injectors / regulator?

 

What do your plugs look like? Running rich?

 

Thanks Hornbrod!

 

Understood; I won't worry about installing this when I get the chance. Out of curiosity, what FP do Bimmers run?

 

IIRC, my issue began before the injectors/FPR swap (about a year ago now)...but I have been noticing it getting worse in the last couple months...slowly, but it is worsening...I'll check the plugs, maybe they will tell me something. I did change them out for stock plugs 3 years ago (25k miles ago). Maybe need to swap the plugs...? I also swapped the distributor and spark plug wires at that time.

 

Thanks again.

[Rohls]

K, a little update.

 

I swapped the Bosch 703 Mustang 5.0L injectors for the new Siemens 22 lb/hr. Starts when COLD a little better....but when at operating temp, still has the 3-5 cranks before it fires up...

 

Still haven't pulled the plugs.

 

I am not running rich from what i can tell.

 

Now what should i try?

 

 

Thanks!

[Rohls]

I ordered an inline check valve to place in the fuel line.

 

Where exactly should I install this check valve? Directly before the fuel filter? Anyone got a pic?

 

Thanks...!

[gogmorgo]

I would put it after the filter, as extra protection so it doesn't get clogged up. However, some filters have check valves in them. In this case there'd be an inlet and outlet marked on it.

[HOrnbrod]

I'd install it in the rubber supply line between the tank and the hard fuel line to keep the supply line as full as possible. Also use fuel injection line hose clamps, not the standard hose clamps.