Running Rich Still but getting close-gctid399452

[hfxjack]

As many of you know I replaced my engine last spring and since then have had nothing but grief. However we are getting close.

Long story short my fuel consumption at trawler speed is the same as running at 3800 rpm. As measured by fuel flow sensor.

When engine is cold I can start easily with no throttle. Same when engine is hot. Engine idles normally. However when the engine is warm say 140F I have to give it 3/4 throttle to get it to start and then I have to keep it around 1000 rpm to keep it running until it is warm > 170F on the gauge.

Now my theory. If the engine temperature sensor ( the one connected to the ECM ) is out of calibration and reads low then the ECM thinks the engine is running colder than it really is and therefore gives it more gas. On my dash gauge running at 3800 temp indicates 185F when at trawler it reads around 170F. A out of calibration sensor will not give a fault because it is not failed but simply not reading accurately.

When plugged into the computer there are no error codes However at idle on the muffs the engine never really gets to normal operating temperature as indicated on the computer and the pulse width on the injectors is longer than normal ( as if the ECM is giving a little choke )

SOOOO I am thinking I change that sensor. Opinions please

7.4L MPI

Thanks

[dktool]

I though you and MM got to the bottom of this last fall, WOW still chasing that problem.

Your a PATIENT man

[Guest]

hfxjack wrote:
As many of you know I replaced my engine last spring and since then have had nothing but grief. However we are getting close.

Long story short my fuel consumption at trawler speed is the same as running at 3800 rpm. As measured by fuel flow sensor.

When engine is cold I can start easily with no throttle. Same when engine is hot. Engine idles normally. However when the engine is warm say 140F I have to give it 3/4 throttle to get it to start and then I have to keep it around 1000 rpm to keep it running until it is warm > 170F on the gauge.

Now my theory. If the engine temperature sensor ( the one connected to the ECM ) is out of calibration and reads low then the ECM thinks the engine is running colder than it really is and therefore gives it more gas. On my dash gauge running at 3800 temp indicates 185F when at trawler it reads around 170F. A out of calibration sensor will not give a fault because it is not failed but simply not reading accurately.

When plugged into the computer there are no error codes However at idle on the muffs the engine never really gets to normal operating temperature as indicated on the computer and the pulse width on the injectors is longer than normal ( as if the ECM is giving a little choke )

SOOOO I am thinking I change that sensor. Opinions please

7.4L MPI

Thanks

John: Sounds like it's extra rich. I wish that I had more experience on your setup. I'll bet that if you can find a tech that's got 7.4 MPI experience, he'll fix it without having to replace a shopping cart full of parts. Good luck, though. Griff

[hfxjack]

dktool wrote:
I though you and MM got to the bottom of this last fall, WOW still chasing that problem.

Your a PATIENT man

Boating isn't as much fun as it used to be. Just ask the Admiral.:thumb

After about two Weeks MM forgot who I was, could not even remember me even when I reminded him of previous conversations. Told me he gets dozens of calls a day how could I expect him to remember everyone. Don't feel it's a MM issue just felt abandoned.

[dktool]

If memory serves, MM was going to verify the cam / valve train that was installed when MAP was potentially an issue, did that happen or was that when they forgot who you were ?

[hfxjack]

No I would have to send engine back. The ECT is a quick and cheap fix. I work with thermisters every day. Over time they will drift certainly due to breakdown at the thermister joint and wide temperature swing encountered with an engine which can be more than 100 degrees.

I tried to find the resistive curve but no luck even if I new the thermister type would help.

In general I am very pleased with the engine especially on plane. If I had to explain it under load the engine is very smooth.

My memory is better than theirs.

Cheers

[Guest]

John,

I'm not clear if you resolved the issue of low engine vacuum at idle or not. What is your idle vacuum now?

Does this engine have a diagnostic port that you can plug software in to read the actual engine operating parameters, or is it just the older type that will allow you to read stored engine codes only?

I hesitate to give troubleshooting opinions without a lot more information than what I see here because I am not familiar with how Mercruiser does their engine management but here's something else to consider assuming that it is a garden variety speed density system.

Engine coolant temperature is normally used in the fueling equation only at start up and up to about 160 degrees F. Analogous to an automatic choke on a carburetor.

I order for the ECM to calculate the desired injector pulse width for a given speed and load it needs to determine the mass of air that each cylinder will be drawing in at that particular speed and load. If it knows the displacement of the engine and it's speed it can calculate the volume of air, but because air mass also depends on air temperature, it also must know the temperature of the air.

If for some reason it thinks that the air temperature is higher than it actually is it tends to lean the mix. If it thinks it is lower than it actually is, it tends to richen it. If you have throttle body injection the air temperature sensor will probably be someplace in the intake tract before the throttle body. If you have port injection then it will probably someplace in the intake manifold itself. Assuming that it is a standard GM sensor, they are very common, cheap and the transfer function is readily available.

Assuming the physical connection to the sensor is good I would expect that you should have 5 volts at the terminals. Check the shop manual to find out for sure. If the applied voltage is correct, then pull the sensor and check the transfer function. If that is within spec then you will have to check the integrity of that part of the circuit back at the ECU to make sure that it is receiving the right signal. You'll probably need the shop manual to find the best way to do this.

The other option is to just make guesses and throw parts at it, but that's not very scientific and tends to get expensive.

[hfxjack]

RJH wrote:
John,

I'm not clear if you resolved the issue of low engine vacuum at idle or not. What is your idle vacuum now?

Does this engine have a diagnostic port that you can plug software in to read the actual engine operating parameters, or is it just the older type that will allow you to read stored engine codes only?

I hesitate to give troubleshooting opinions without a lot more information than what I see here because I am not familiar with how Mercruiser does their engine management but here's something else to consider assuming that it is a garden variety speed density system.

Engine coolant temperature is normally used in the fueling equation only at start up and up to about 160 degrees F. Analogous to an automatic choke on a carburetor.

I order for the ECM to calculate the desired injector pulse width for a given speed and load it needs to determine the mass of air that each cylinder will be drawing in at that particular speed and load. If it knows the displacement of the engine and it's speed it can calculate the volume of air, but because air mass also depends on air temperature, it also must know the temperature of the air.

If for some reason it thinks that the air temperature is higher than it actually is it tends to lean the mix. If it thinks it is lower than it actually is, it tends to richen it. If you have throttle body injection the air temperature sensor will probably be someplace in the intake tract before the throttle body. If you have port injection then it will probably someplace in the intake manifold itself. Assuming that it is a standard GM sensor, they are very common, cheap and the transfer function is readily available.

Assuming the physical connection to the sensor is good I would expect that you should have 5 volts at the terminals. Check the shop manual to find out for sure. If the applied voltage is correct, then pull the sensor and check the transfer function. If that is within spec then you will have to check the integrity of that part of the circuit back at the ECU to make sure that it is receiving the right signal. You'll probably need the shop manual to find the best way to do this.

The other option is to just make guesses and throw parts at it, but that's not very scientific and tends to get expensive.

We have had a computer on and can read system functions as stated it tends to read low for engine temp at idle never really comes off of what you would call choke. The inlet temp reads correctly. You touched on the 160F temp as it related to chocking the engine and that is my theory. On plane my dash gauge and who knows how accurate that is reads 180 to 185 at trawler around 170. Since I have a new 160 stat and new clean and serviced cooling system it is reasonable to assume my dash gauge is off and in reality running cooler than what my dash gauge says. Running fuel burn seems normal and consumption for a trip compared to other years is the same. However at trawler burn is high so if ECT is telling ECM that engine is say 145 150 the "Choke" is on. I don't believe we ever saw an engine temp at idle on the computer above 160.

With regards to vacuum my mechanics feel it is a result of the running rich not the cause of running rich. Tested and retested for leaks. All vacuum lines are new. Had the ECM checked by Mercruiser twice and have tried another one.

There are no error codes. The pulse width is on the high end of normal and engine temp indicates a little low.

[Guest]

Again, without knowing exactly how the Mercruiser code handles warm up enrichment, I'd have to agree with you and suspect that the ECM thinks the engine is not up to full operating temperature yet, whether it's real or not. I would trust the temperature sensor before I would trust a dash gauge

With a 160 degree thermostat the engine should come up to 160 before the thermostat opens at all, even at idle, and cycle between about 165 and 155 give or take a bit. It might get a bit warmer when loaded up depending on how your cooling system works.

I suppose the way to attack it might be to first determine just what the actual operating temperature is. An infra red heat sensing gun would probably be the easiest, shooting the thermostat housing. If readings are less than the thermostat rating you have a mechanical issue. If it is very close to the rating you have an electrical issue.

If I recall your vacuum at idle was quite low. I have trouble imagining how a rich mixture would cause that unless it was extremely rich. That is rich enough to cause blackish exhaust smoke.

If the sensor is a standard GM unit I have the transfer functions for it but it is an Excel file and I don't think I can attach it. If you want it PM me with your email address and I'll email it to you.

[dktool]

If I recall your vacuum at idle was quite low.

That is where I thought you left off last fall, that's why I brought up the cam / MM point.

[hfxjack]

I purchased the sensor but not yet installed. I was on the water so did some testing. At 170 on my gauge warming engine from cold first start of the day idle 600 rpm's I was burning 3 gph. Ran on plane for 6 miles engine temp 185 stopped at idle was burning 2 gph. Ran again slowed to trawler for about 10 minutes temp was 175 stopped went to idle fuel burn was 2.7 gph.

Engine cold warming up fuel burn was 4.5 to 5 gph

[hfxjack]

dktool wrote:
That is where I thought you left off last fall, that's why I brought up the cam / MM point.

I was but the mechanic took it for a sea trial and was very confident it was not the cam. Another point I forgot about sorry it's been so long perhaps I should write a novel and recoup some of my money .

When hot and running on plane if I pull back to neutral quickly it stays running. If at trawler for some time and do the same it stalls. You can hear the stepper motor sorry forgot the name of it but in the old days it was the dash pot, stepping trying to catch up but it cannot open fast enough. Also it does behave as it should has been replaced and conforms to specs. It's just that the fuel delivery is so rich the air can't catch up on rapid throttle shut off. Hey I remembered something else!!! when hot coming off plane go to neutral engine runs smooth at 600 rpms if at trawler for a while and go to to neutral then engine rolls rpms jump between 600 and 800 makes for great docking at times. As a result my docking skills have greatly improved.

I have asked Robert for the resistance curve then I can check the resistance to the curve at different temperatures and confirm once and for all. It's too bad this information is not readily available. More marine industry BS.

[Guest]

John,

Those fuel consumption numbers look way too high to me, even if you are running very rich. What are you using to measure consumption?

If I were you I would spend the ~$650 and buy your own Diacom diagnostic software from Rinda. You are fortunate to have an engine that it will support. That way you can plug it in, go for a ride and data log all engine operating parameters. You can then analyze the log to see what areas don't look right. I think it even has a fuel consumption read out as well which will be very accurate. All you have to do is eliminate a couple of trips to the shop and it will have paid for itself.

[Guest]

Not knowing your exact system, I can tell you that all fuel injection systems share pretty much the same way of measuring and correcting fuel mixture. The most important devices are MAP(manifold absolute pressure), ECT(engine coolant sensor) and O2(Oxygen Sensor or Lamda).

A vacuum leak to the MAP, or bad MAP, Bad ECT sensor, or Bad O2 are the most common causes of rich mixture. Other than that we have seen bad connectors to any of these sensors. Almost never is the ECM the culprit.

[hfxjack]

RJH wrote:
John,

Those fuel consumption numbers look way too high to me, even if you are running very rich. What are you using to measure consumption?

If I were you I would spend the ~$650 and buy your own Diacom diagnostic software from Rinda. You are fortunate to have an engine that it will support. That way you can plug it in, go for a ride and data log all engine operating parameters. You can then analyze the log to see what areas don't look right. I think it even has a fuel consumption read out as well which will be very accurate. All you have to do is eliminate a couple of trips to the shop and it will have paid for itself.

Robert I use a lowrance fuel flow transmitter. I trust it in that fuel used on a trip as reported and fuel fill are within a few litres always. I should be burning 1 - 2 USGPH at idle. per the user manual. .650 of a boat unit may be tough with the Admiral.

Cheers