Hard Start When Warm

[Phillbo]

Hi all~

Engine is a 2004 6.2L MPI Merc with about 450 hours (if that matters).

Issue is that is starts and runs great when cold but once it gets up to temp it just cranks and will not start unless I give it almost full throttles.

It sounds to me like it is starving for air when warm but then i am not a mechanic. I told my mechanic this and he checked the IAC and it checked out alright. It is not throwing any codes.

I just had a full tune up done and no change. Any suggestions where to look next?

[ss3964spd]

My first guess would have been the IAC but since that supposedly checks out....

Perhaps temp sensor or throttle position sensor. Both feed the ECC and if either are providing bad information the computer will map out a fuel mixture for a condition that doesn't actually exist. Might also be the fuel pressure regulator or possibly the fuel pump - both are easy to check with a pressure gauge.

Dan

[rodrigo]

I agree with Dan on the fuel pressure regulator....good luck!

[The Web Two]

LT. Phillbo, Did the fuel Pressure Regulator fix the problem? I have a 2001 Maxum 3000 SCR with Duel 5.7L engines. Same problem. It will start up and run fine cold, but will not start after sitting in the 1/2 day. It seems to be fuel related, since squirting some starting fluid get's it to pop a few times. Also I notice that occasionally the RPM gauge will jump up by 1500 rpm's, the engine speed did not change.

Please let me know if the pressure regulator fixed your problem.

Thanks The Web 2

[ss3964spd]

TW2, Are you having problems getting both engines started? If so I'd be looking for something other than the fuel pressure regulators (I'm assuming the engines are fuel injected) - as I rather doubt both FPR's would go bad at the same time. As for the tach jumping - are both of them jumping?

[The Web Two]

Sorry I did not get back to you on this until now. I am quite the novis on how to use the threads and forums. So please bare with me.

Anyway, to answer your questions. Sometimes the issue is with both engines, some times with just the starboard side. The starter will crank, but it will not fire. When I squirt some starting fluid in, it tries to start. I have picked up a fuel pressure gauge and was going to check it out this weekend.

On the tach it is only on the starboard side. It is not jumping. It stays high. When they both start and run, if the port is idling around 800 RPM, then the starbord will be "reading" 1200 RPM. I know by sound of both engines, it is only idling around 6-800. Unless you have another suggestions, I am going to try a new coil to solve the tach.

[ss3964spd]

If the engine(s) seem to be running normally (once they are running) replacing the coil(s) won't solve your tach reading issue. You have either a wiring problem - probably ground, for the questionable tach or the tach itself is going bad. Are your engines carb'd or FI?

[shrew]

Originally Posted by ss3964spd

Are your engines carb'd or FI?

Originally Posted by The Web Two

When I squirt some starting fluid in, it tries to start.

I'm guessing Carb's.

[The Web Two]

Guys, I spent most of the day Saturday in the engine compartment. First, to answer your question, these are 2001, 5.7liter Fuel Injected engines. The port engine starts and runs fine. It just has a problem with the tach. I will look for a ground issue to solve that.

The starboard engine will not fire the fuel injectors. It cranks and will pop when sprayed with starting fluid. I have swapped the ecm's, fuel pump relay and fuses. I pulled the wiring from the fuel injector and shot 12 volts to it and they both spray. Plenty of fuel pressure. When the key is turned to on, you hear the fuel pump for a few seconds, then it shuts off.

Looking at the wiring diagram, I am seeing the fuel pressure switch. I just can't find it on the motor. There is a cover over the fuel pump, I am guessing it is under there. Not sure. I thought about the distributor module. It may be the culprit, however, I could not figure out how to remove the rotor to get to the module.

What could be causing the fuel injectors, not to fire?

[mmwjr]

WEB,
when you first turn the key you will hear the fuel pump pressurize the system. It is still on just much quiter. You said with starter fluid it's pops, then the module in the distributor is firing the spark plugs. BTY this module has nothing to do with the injectors. The ECM controls the injectors. Since you have swapped them I will assume it's Ok. This leaves all the sensors that the ECM gets engine data from or a break in the wiring harness. From your discription that they both spray it sounds like you have TBI (2 next to each other on what looks like a carb) and not port injection (injector at every cylinder).
I'll see if i can find some time to do a little reading and come up with any ideas.

[The Web Two]

Thanks Mike. I appreciate any and all help I can get. You are correct, there are two injectors sitting on a thottle body. I don't get why they will not fire. I suppose I can start checking wires in the wiring harness. I suspect it is some other sensor. Just don't know where else to look.

I have been looking on line and can't seem to locate the fuel pressure switch. In my "owners manual" there is a wiring diagram that calls it out. They just don't say where it is located. I can't seem to find one on either motor.

Bill

[mmwjr]

Looks like fuse (16) provides 12V to the injectors so I would check this. If fuse is OK then verify 12V is getting to fuse with it removed, should be 12V on the red wire. let me know what you find.

[ss3964spd]

When you say you get a "pop" when you spray starting fluid in it what exactly does "pop" mean? Does it mean that it seems like the engine actually tries to start? All I'm getting at is we're assuming the ignition system is working but it hasn't actually been verified (unless I missed it). That the injectors aren't firing is a symptom but it's possible that the ECM is shutting down the injectors because it isn't seeing a signal from the ignition. Or not - I don't know for certain.

Have you attempted to start the engine with the throttle partially open? If it will start like that it would indicate the TPS isn't playing nice. I'd also consider by-passing the fuel pump relay.

[The Web Two]

Thanks guys, I will actually pull a plug and see if she sparks. The 'Pop' may be just that. Still no spark. First I will check the TPS to make sure that is not the bad seed. The fuel pump relay click's, and we hear the pump when the key is turned to on. This means the fuel pump relay is working. If still no spark, then I go to the distributor module.

Plan on working on it tomorrow, independence day. I will let you all know what I find.

Thanks
Bill

By the way, it won't crank if the throttle is not in the right spot. What does this tell you? Not sure, but I will still check the TPS.

[mmwjr]

W2, you are loosing me. Started with no fuel, then pop, now, no spark. Oh yea cranking issues.

1. If no fuel then can't pop unless you add fuel or starting fluid into engine.

2. If really popping then you have spark. But test it anyway.

3. Position sensitive cranking, sounds like he netural saftey switch on the shifter. See if wiggling it back an forths makes a difference. if it does it is probably the switch.

4. I agree it sounds like the fuel pump is running. may need to check it's output pressure.

let us know what you find.

[The Web Two]

Guys, Thanks for your help. I worked on it yeasterday morning. Happy 4th...
1. The engine would crank, but not start. When I squirted it with starting fluid, it would try to run. 'Pop' Therefore, I thought I had spark and no fuel injectors firing.

2. I pulled a spark plug and used a jumper cable to insure a good spark plug ground. NO SPARK. The popping must have been starting fluid diesling.

3. With no spark, I went directly to the distributor module ( since I had previously changed the ECM and Coil ).
Great news, she fires up and runs like a top.

Moral of the story: Given good compression, starting fluid will make an engine run even without spark.

Thanks again.

Bill

[mmwjr]

Glad to hear you fixed the problem.

Enjoy the rest of the season!

[ss3964spd]

Nailed it!!

Glad you got it sorted out Bill. Have fun out there.