Idles and runs in service mode

[jlewis12345]

Engine: 2000 4.3 Mercruiser EFI S/N 0L660295, Thunderbolt IV ignition I think, MEFI3 ECM


Initial problem started with a cracked head (cracked on the exhaust port). Finally diagnosed that and have it back together with a re-manufactured head. Timed the engine in service mode (8 degrees ), seemed fine. Took out of service mode idles right at 600 rpms.

Now the problem is engine stumbles and runs horrible if rpms get above idle. Engine backfires, through the intake, at around 1500-2000 rpms. I have put the timing light on it while running rough, and the timing marks are not visible. I am assuming it is advancing too far. I have double checked the timing in service mode a couple of times it is still at 8 degrees. I have double checked the plug wires by tracing and then again by a continuity check. I am not getting any codes from the on board diagnostics. I have replaced the IAC valve because it failed the test in the manual (idle did not increase by 200 rpms when disconnected). The new IAC valve changed nothing. I checked fuel pressure, it is holding 30 psi.

Few side notes:

-The rpms did not automatically go to 1200 rpms when put in service mode. They went to 1000 rpms. I manually accelerated it to 1200 rpms for setting timing.
-I checked the vacuum level at idle and it is about 13 psi, which is below the 15 psi minimum in the manual. The Throttle body has a louder vacuum noise than I remember (hissing), and the throttle body is cold enough to cause condensation on it when running. I checked for leaks by spraying carb cleaner around the throttle body and intake while the engine was running. It didn't cause any change in the engine.
-when in service mode it seems to be running good.

I'm out of trouble shooting ideas. Any help is greatly appreciated. Thanks

 

[Bondo]

Ayuh,.... Sounds like it's runnin' Lean,..... probably why the head cracked,...

 

[jlewis12345]

Thanks for the help.
If it is running lean how would I fix that? Isn't that controlled by the ECM. Clean the injectors?

 

[Scott Danforth]

Engine: 2000 4.3 Mercruiser EFI S/N 0L660295, Thunderbolt IV ignition I think, MEFI3 ECM


I checked fuel pressure, it is holding 40 psi.

fuel pressure should be a bit higher.

check your regulator, check the fuel pump

here is a link to manual #25 http://boatinfo.no/lib/mercruiser/manuals/mercruiser25.html#/0

 

[Bondo]

Ayuh,.... I'd examine the Contents of the fuel filter 1st,....

It could be crud in the gas, in the tank, or 'bout anywhere in the fuel system, restrictin' the flow,.....

 

[alldodge]

You have MEFI-3 and TBV, fuel pressure is high and should be 30 psi.

re-manufactured head. ... backfires, through the intake .... vacuum level at idle and it is about 13 in Hg

Think there is am issue with a valve or valve seat. Do a compression and leak down test to find out which intake valve is not fully closing. Also suggest taking the regulator off, clean and clean the screen (item 16)

http://www.mercruiserparts.com/bam/subassembly/31497/5388/60

 

[jlewis12345]

I will look into the fuel system

I've done a compression check. Numbers are 100-115 psi. Not the best numbers but also not sure how accurate my gage is, but none of the cylinders are showing really low compression. The side with the Remanufactured Head has the better numbers 110-115.

I did a leakdown check on the side with the remanufactured head. It checked good 10-20% leakdown if I remember right.

What I don't get is, wouldn't a mechanical problem such as valves or fuel also show up in service mode. It is night and day the way it runs in service mode and the way it runs out of service mode. From what I can tell the timing is way out when not in service mode.

Thanks for the help.

 

[alldodge]

Low compression can give you low vacuum numbers

The big issue I'm reading is backfire thru the intake, this happens when a intake valve is not closed most often. Can also be caused by distributer cap tracking from deposits inside.

When placed in service mode sensors such as MAP and ECT are disabled, IAC is opened a calibrated amount and timing is locked. When taken out of service mode the sensors go back to work.

Need to scan for codes using a reader or resister led

 

[Bondo]

Numbers are 100-115 psi. Not the best numbers but also not sure how accurate my gage is, but none of the cylinders are showing really low compression. The side with the Remanufactured Head has the better numbers 110-115.

Ayuh,.... Those numbers say it's a very tired motor that's ready to be rebuilt,....

oughta be 150 psi or better,...

 

[alldodge]

Did you clean the bore where the IAC is at, along with cleaning the IAC muffler?

When its running remove the oil fill cap and see if any vapors are puffing out or if air being drawing in

 

[jlewis12345]

AllDodge,
Sorry for the bad information about fuel pressure. I re-checked it. It is right at 32 psi when running. I edited the initial post to correct this. I also pulled a vacuum on the regulator while idling and the fuel pressure dropped by 5 psi as described in the manual. So I think the fuel system is good.

I don't have the IAC muffler, not sure if I never had it or if it fell out while removing/installing intake. From what I read the IAC muffler is to reduce noise and shouldn't really affect engine performance.

I opened the oil cap while idling, no vapors or airflow.

Bondo,
I am blaming the low values on the compression test on a weak starter. It is not old, but just hasn't turned the engine over very fast since installation at the beginning of the season. Would low compression cause the timing to move around, and backfiring?

New Results
I have tried unplugging the MAP sensor and ran engine without it (should use a set value according to manual). This did not change anything.

 

[alldodge]

. It is right at 32 psi when running. I edited the initial post to correct this.

This is actually what folks worry about with long edit times, editing an old post, it should actually stay the same.

I don't have the IAC muffler, not sure if I never had it or if it fell out while removing/installing intake. From what I read the IAC muffler is to reduce noise and shouldn't really affect engine performance.

Agree on both counts but also keeps crud out of the IAC chamber.

I opened the oil cap while idling, no vapors or airflow.

This could be good or bad. Good if the motor is not burning any oil and such, but bad if the intake manifold is leaking on the under side

New Results
I have tried unplugging the MAP sensor and ran engine without it (should use a set value according to manual). This did not change anything.

Unplugging the MAP causes a rich fuel mix, same as if the throttle is WOT. Will also set a code.

Check for 5V with key ON and not running.

 

[jlewis12345]

I'm getting 5V at the MAP sensor.

New Problem
One of my fuel injectors just quit working. I tried switching the wiring harnesses between the good injector and the bad injector it didn't change any thing. I removed both injectors from the throttle body. Plugged them back in (one at a time to the same harness) and held them while someone else tried to crank the boat. I could feel the good injector working. The bad injector did nothing.

At this point I'm confident the injector is bad. Is there any disassembly that can be done? Is there an alternate to the Mercruiser injector [FONT=&quot]852956A 1 (this one is pricey)?

Also not sure if the injector is related to my original problem or not, but one problem at a time.

Thanks[/FONT]

 

[jlewis12345]

Found this set of injectors on ebay. Seems a lot more reasonable than new.

 

[alldodge]

You can get yours cleaned and flow tested for $20 each. Sure one may be not fixable but it may be just fine, don't know. The ebay injectors will work IF they have been cleaned and flow tested properly. That said the guy says he is No 1 guy doing the job, but maybe he is and maybe not.

What you want to know is were the injectors flow tested to at least 5000 rpms.There are many injector places that will do it on line. Guess the good thing with ebay, is if they don't work they can be returned

 

[jlewis12345]

Replaced the fuel injectors. They work now, but engine still backfires and stumbles above idle. Timing marks are not visible above idle.

Tried it again in service mode, timing is right at 8 degrees BTDC. I can't figure out what would make it rung good in service mode (up to 3500 rpms, that's as high as I tested it), and not run right when out of service mode. It seems to be related to the timing advance. So, either the computer is getting bad inputs from the sensors or the computer is giving bad output. Am I missing something? Running, seemingly good, in service mode means mechanically it is good right?

 

[alldodge]

Backfire and stumble above idle, causes:

Injectors do not fire consistently
Injector sticking or clogged
Bad wire connection between ECM and 12V power

Timing is off
Not set at 8 BTDC (maybe 8 ATDC, or other)
Timing chain slipped

Valve Train
Valve warped or bent stem
Spring broken
Valve guide worn
Bent push rod or worn rock arm/studs

Distributer
Cap tracking inside

ECM and Sensors
Bad connections to ECM and its sensors
12V and Grounds along with 5V connections

 

[jlewis12345]

Thanks for the assistance. Issue has been resolved.

Replaced two items. The distributor cap and the module underneath the distributor cap. Running good now.