Different Manifold Temperature and Fuel Consumption

[kerslaks]

I have recently noticed a difference in Manifold Temperature and Boost Pressure on my boat.

I have QSB5.9 380s on a Sea Ray Sundancer 390.

The SmartCraft indicates no other differences but the fuel burn is higher on the cooler engine. That seems odd to me.

I burn .5gph more on port, but the manifold temperature is 30 degrees cooler than it is on the stbd. (Port is 100, stbd is 130 at 2600RPM)

Does anyone know:
- what the nominal temperature is?
- why would the engine with the higher temp burn less fuel?

The port aftercooler has been cleaned, not the stbd. If I clean the stbd aftercooler, should I expect an increase in fuel consumption?

 

[kerslaks]

I have learned from a post on another website that normal is 70F - 100F. And at 70F is the upper limit.

So it would seem that the stbd engine manifold temp is too high and that I need to clean my aftercoolers.

Probably both of them since the Port is at 105F, which is also little high.

Anyone have tips on how to get the stbd aftercooler out of its hole on an '05 390?

 

[BonBini]

If you had Cats I could give you some specific specs from sea trial tests. Cummins I can only talk basic diesel engine. Hopefully a Cummins Tech will chime in.

Typically when you have one engine down on power, the sister picks up the load. I have been out on power complaints many times when the complaint was xxx engine black smoking, the problem ends up being the sister with a dead hole/low fuel pressure/...

Not saying your strb has a problem just yet!... You say your port fuel burn is up. How does the boost pressure compare between the two? Also do you have instrumentation to read fuel pressure?

 

[kerslaks]

Boost pressure is higher on the stbd engine too.

No fuel pressure but fuel flow is higher. This was confirmed by use. The port engine draws fuel from the port tank faster than the stbd engine draws from the starboard tank.

The SmartCraft reports pretty close matches on transmisson load, temperature, and pressure.

 

[BonBini]

2600 rpm..... Port ....strb
Fuel……........up .......low
Boost……......low ......up
Man temp. ..100 ......130

Do you have any real #’s for boost pressure?
Is there any generator use associated with fuel use?
SC reports trans loads matched: what type of sensing is used to figure this?

 

[kerslaks]

Your table is correct.

I'll have to get those numbers the next time I run the boat. I didn't think to write them down and now I am uncertain.

Thanks!

 

[BonBini]

Next time out, maybe you could try a power test on each engine. Cat has a procedure where at wot, you feather back the the throttle on one engine to put her sister into a lug. The idea is to get the sister down on her knees to a peek torque rpm and then take readings. A diesel at peek torque rpm is at max fuel setting with highest exhaust flow and spooling the turbo at it’s max. Things heat up quick, so you don’t want to stay in a deep lug long… I could probable dig up the Cat procedure and PM it to you if the above isn’t clear enough.

From looking at Cummins/Mercruiser power ratings. Looks like peek torque for you is 2000 rpms. May be difficult to get one engine down that low. I don’t know. But if you can get them down some what for a equal comparison, at least it should take props out of the equation.

 

[kerslaks]

Aren't the props still the main load on the engine?

And if the props are different, and the rpm is the same, won't the load be different?

 

[BonBini]

Aren't the props still the main load on the engine?

And if the props are different, and the rpm is the same, won't the load be different?

*Yes the props are the main load. They are actually somewhat of a water brake dyno and can be used as a tool.

*Yes if the props are different, the engines will be loaded differantly when at a syncronized cruise rpm.

As a basic not too technical test: Under wot condition, if both engines produce nearly the same boost pressure when individually tested, that alone is a good indication that both engines are closely matched.

In your case with different boost pressure readings at synchronized cruise rpms: on one hand... one engine may be down on power resulting in more governor to keep up with prop demand. On the other hand... one less efficient prop can result in less governor to match rpms of the sister engine… So by dyno testing each engine individually, your taking any prop variation out of the equation.

I'm thinking with your different readings, testing the engines under the same conditions may be the place of referance.

Having said all that. I certainly hope these Cummins don’t have waist gate turbos. I’ve never seen one on the older Cats I’m more familiar with!

 

[sbmar.com]

The QSB 380 does not have a wastegate.. The engine uses the same turbo as the mechanical 330/370 Diamond..

As to our posters possible issues, he may have aftercoolers that needs proper servicing--That could or would account for different IMT reading .. Air clearen condition ?? If he has WalkerAirsep, all bets are off..

http://sbmar.com/Maintenance/Aftercooler_Maintenance.php (Cummins B, C, & QSB)

His issue may also be gauge reading inconsistency / tolerances, normal engine tolerance in performance ( 5% in this case), slight difference in the gear ratio from Port to Starboard gears, slight difference in props (are they Class 1 or better ?) or any combo of the above..

Not really enough posted to make a proper diagnoses.. In most cases, "Marine Age" that affect the saltwater cooled aftercooler is way more important to look at than actual engine hours...


Tony

 

[kerslaks]

On Saturday I replaced both raw water impellers. I had suspected they were the originals since the paint was unbroken on all of the bolts. They were.

All of the vanes were intact, but they had definitely taken a set. Both impellers looked to be in the same condition.

Ran the boat up to normal operating temps at the dock under no load. I only went to 1600RPM but everything matched between port and stbd. The boost pressure was only .5psi at that RPM, so not a lot of info there.

I'll take it out Thursday afternoon take some measurements at different RPMs. And I'll try out a load test.

I suspect fouling in the after cooler more than anything else. A difference in the props is another possibility, but I would have seen that from the start. What I am seeing seems to be a more recent issue.

What is really odd to me is the difference in fuel consumption seems reversed.

I did notice that I had a lot of "blind mosquitoes" in my intake air filters. They could definitely be effecting the flow of air into the turbo. They get cleaned next.