1999 Mercruiser 350MAG Issues

Ok. I did some side by side comparisons and I've uploaded two images of the data. The left log is the port engine and the right the starboard. The file named NoSpool file is when the issue happens and at WOT, it won't spool up. The Spool file is another point in the same log file where the engine did, for a short time, spool up.

The first thing that struck me is that first off, the IAC values on the SB engine at high throttle is always stuck at 150. I assume that is one of its detents. I'm not sure, however, if that is open or closed. When the coolant sensor had a low value (cold engine) and at idle, the IAC value was in the 70's. The IAC equates to choke right?

The second thing that struck me is the BPW (which I assume is the injector base pulse width...amount of fuel). When the engine spooled, the value was noticeably higher than when it didn't. That supports the behavior I see. When the throttles are mashed and it doesn't spool up, the engine noise most definitely is higher...like the intake is opening and it wants to go but is being held back...not getting the fuel it needs.

Lastly, the GPH is higher when it did spool up. The BPW was higher on the SB at that time. That would be a rich condition right? Would that explain the slightly less RPMs?

So does this point to the problem? If so, what do you think the causes could be? It looks like the computer is not doing what it is supposed to be doing. What would cause it to act this way?

Thanks for any input.

Jon
 
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IAC is idle air controller. It sets the idle. Like the fast idle cam on a carb, it runs the engine a bit faster when cold.

BPW is derived from a map in MEFI-3's memory. It uses throttle, temp, RPM, MAP (manifold absolute pressure), etc to find the length of time to open the injectors. It's a base number since actual operating conditions will modify it within certain parameters, which are also part of the map. Since these are no-feedback engines, the BPW is used directly. If the engine had O2 sensors the feedback from the O2 sensor would be used to correct the base number. If the correction was more than, say 10%, it would not allow more than that amount of correction and flag a "check engine" condition. MEFI-3 doesn't do that, so we concern ourselves just with BPW.

There's no smoking gun that I see here. When the engine is turning faster it gets more fuel. That make sense. What doesn't make sense is that the TPS is showing wide open, MAP is high so the engine is getting plenty of air, why isn't the fuel flow commensurate? The only thing I see is that the timing is slightly retarded. Don't know if the system is showing detonation, which will bring back the spark a bit. I don't see it but I'll think on it for a while.

Best regards,
Frank
 
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Guys.........I am in over my head on this one in terms of providing many suggestions. One thought I had though was to swap the ECM's engine to engine to see if the problem stays with the same engine or moves to other. Any thoughts on this?

Dave
 
Don't think we're going in the right direction. I know that you already had the MEFI-3 PCM swapped, so probably there is nothing left in the EFI system that's not be replaced other than the wiring harness. I've seen bad wiring harnesses, but your's does not feel like one, yet. I'd go back over plowed ground and have the compression readings rechecked, especially since the problem seems intermittent. Also I'd recommend checking the timing chain. It's not hard to do, but it is a bit of work.

To check the timing chain, pull all 8 plugs and remove the distributor. Now put a wrench on the damper pulley and observe the distributor drive gear. Turn the wrench backwards until the distributor drive just barely starts to move. Now turn the wrench forward until the drive just starts to move and measure the angle between back and forward. I have to lookup the spec, but it should be tiny. If the chain is worn, then you can get funny performance.

Best regards,
Frank
 
Do you mean remove the entire distributor? Or just the cap and watch the rotor?

Speaking of the distributor, when I changed the distributor pickup sensor, I noticed a small amount of oil under the origional sensor (which I'm putting back on and getting my $50 back for the one they suggested buying). It didn't completely cover the bottom. There was just a small amount.

I'm building a second cable so that I can log both engines simulteneously. That way the conditions will be identical. I'm also going to log from cold start as well to see how everything behaves.

Unfortunately, I also have to buy around 60 gallons of fuel....yuk!

Thanks!

Jon
 
Yes, with the distributor in and cap off. Sorry I was not clear on that.

The manual does not give a reference for number of degree that the crank should move back and forth without moving the camshaft. If it's more than a little, you'll have to pull the front timing case cover and measure it. That's kind of a bitch to do. I did it and didn't enjoy it.

Best regards,
Frank
 
I've been away for a bit. Sorry no updates. Thanks again for everyones help. Hopefully this weekend I can resume diagnostics. We did go down this weekend but it was too hectick to have the engine compartment open. I did get to buy $340 in 2 week old E10 though? :-(

Jon
 
Have you contacted the Mercruiser tech rep yet? One is coming to my boat to help with a problem that no one could fix. I had to watch over the mechanics shoulder to see that the ECM wasn't receiving all the necessary data to run smoothly. So now they're going to troubleshoot the wiring harnesses.

I think it's at a point where you should have the rep look at your issue. I spent over $2k between the dealer and a local mechanic before doing that, and I wish I would have done it earlier...

Eddie
 
Well, it's been frustrating because I haven't been able to spend much time on the engine. You know...family time and all comes first. I did, however, get a second cable build and have ScannerPro working on both engines simultaneously. Unfortunately, I screwed up my initial test and only saved the SB engine log. For those interested, I'm uploading my logs. The NoWake logs are before and after my test runs. You will see that this time, even at full throttle, the engines didn't even exceed 3000 rpms.

I'm going to do the tests again this weekend and have a better side by side comparison. As always, any ideas are appreciated.

Eddie, I'm probably going to looking at doing that. To date, I've not really spent any money on it. My dealer has just essentially abandoned me. His proposition was that if I brought the boat to their marine, he would take one more look at it. Well, it would cost me over $500 in fuel to get it there. So he knows I won't do it. So he gave himself a way out.

So frustrating... Hopefully this weekend I can do a clean test.

Thanks.

Jon
 
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Jon

After months of frustration and expense replacing parts I have solved my problem. I dragged the mechanic out with me last week and got him to diagnose everything while under load. At the end of many runs up and down our river the diagnostic equipment used showed nothing. Prior to heading back to the pen he tried advancing the timing and bingo I got my rpm's back. I'm not sure but but I think he advanced the timing by about 5 degrees. At 4500 rpm she ran like a clock with no signs of pinging. His explanation was that with a bit of age a bit of slop occurs hence the need for the adjstment. Really I don't care I'm just happy to have two motors providing the same output.

Not sure if you want to give it a go but if you got to the point like me where you would give anything a go its worth a try.

Let me know how you go?

Wayne
 
Jon,

The Mercruiser rep comes to your boat for free. He has a computer and that's about it I think, but if you have some tools, he'll tell you what to do to test certain items. I had my mechanic turning wrenches, but you might be able to do it. It's worth a try. I'm not sure if you have to go through a mechanic to get him out to your boat, but he found my problem in 5 minutes after 7 months and 4 mechanics couldn't....

Eddie
 
Wayne, I'm most definitely at the same point. I'll give that a try. 5 degrees certainly isn't going to hurt anything. Thanks for the reply!

Eddie, how do I get in touch with a Mercruiser representative? Who did you contact? Thanks for reply as well!

Jon
 
My mechanic called the Mercruiser rep for me. For the Mid-Atlantic region, the rep is Michael Anthony. Here's Mercruiser's phone # - 405-743-6566. Press 4 for Stern Drive applications, and maybe they'll hook you up with the rep. If not, your mechanic would have to call him.

Eddie
 
I have remans (warantee replacements). Is it possible that the timing marks are wrong. The engine doesn't have the guage like I've seen in manuals. Just two marks on inside and one mark in motion. Thinking back, the engine ran fine before they had the top end off. Does the distributor have to come off with the intake and heads? If so, if they "properly" set the timing afterwards, that might explain the change in behavior. Maybe I just need to play with the timing.

Thoughts? It seemed to make a difference for Wayne.

Hopefully I'll be able to play with it this weekend. My wife already has people coming down so I'll have to squeek it in Saturday morning.

Jon
 
Ok...first off, I'm estatic because 5 degrees of advance made it run like a top. We played with the timing above and below that and that was the magic number...save as Waynes. No acceleration or top end problems. Temperatures were dead on.

Even though I'm very pleased with the outcome, I'm perplexed. We verified that it was at -8 before doing anything and it was dead on. What in the engine could be 5 degrees off? Seems any gear being off would cause it to be off more than 5 degrees. One thing to note is that even now the computer still shows almost a 5 degree spark advance difference between my port engine and this engine. This engine is 5 degrees less. So it seems all we did was compensate for some other issue.

If you don't think I should care I won't. But it just seems odd to me.

Any thoughts?

Thanks.

Jon
 
I just went through some logs and noticed that the numbers are off as I was concerned about. I have included a snapshot at steady state cruise RPM. The advance difference seemed to average around 3 degrees on this run. So 5 might be too far. But temps were down and there was no knocking...so who knows. My key concern is the TPS, which correlates to the physical position I see at the controls and the BPW which is in fact causing almost a 3 GPH difference. So does that mean it's running rich? Does that possibly explain why I had to advance the timing?

Thanks.

Jon
 
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You have more air and more fuel running though that engine. Note that the MAP is higher as is the TPS. It's probably not excessively rich as the engine seems to be taking more air. The problem is, you don't know what you don't know. Why is one engine taking more air and fuel to make the same power as the other engine? It's about a 20% difference. If you have one weak cylinder, you'd be 12% down on power, so 20% could be a big deal.

Best regards,
Frank
 
I've been pondering this and I keep coming back to the TPS and MAP. What would make the MAP different? What is the MAP really for that matter. I know what it stands for but what does it really mean? The pressure in the intake manifold caused by the incomming air? Could a leak in the intake manifold account for a higher MAP value?

One test I want to do this weekend is matching the TPS values. I keep trying to sync the engines but haven't sync'd the throttles to see where the differences are in that situation.

I think I am going to sell this thing and go back to a carburated engine. Yeah they are finiky starters...but they are easier to diagnose.

Jon
 
I do not think you have an EFI problem. MAP is Manifold Absolute Pressure. The ECM uses MAP, TPS, RPM and the known engine size to calculate the amount of air flowing though the engine so that the proper amount of fuel is injected. The higher MAP and TPS shows that the throttle is opened more, so the engine should be making more power. But it's not. So since the EFI is doing it's job, something else is not.

I would do three things
  1. Compression test
  2. Leak down test
  3. Power balance test

Compression test is a quick check of engine health. The leak down test is like the compression test, but a bit more precise. You can pinpoint bad a bad intake, exhaust, or rings with a leak down test. Power balance is also easy. You pull one plug wire at a time and see if pulling one wire makes no difference or little difference in engine performance. With electronic ignition, remember to ground the plug wire when you disconnect it. Don't leave it just disconnected or the amplifier will be damaged.

Best regards,
Frank
 
Yeah, I'm going to do a thottle sync test this weekend as well. The thought pattern I was going down was that the amount of air moving, unless the intake is restricted, would be relative to the RPM. Although opening the throttle allows more air and directs the computer to inject more fuel, it would take an increase in RPMs to actually draw more air.

Could the higher MAP value indicate a leak in the intake manifold? It has been off twice since they started working on it.

They told me they did a compression check but that may have been before they had the heads rebuilt. I'll have to see if my buddy has a compression guage (or else maybe I'll have a new tool in my belt). They also replaced cap, rotor, wires, and plugs.

Thanks Frank for the reply!

Jon
 
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