6.2 MPI surging at idle

Need Advice

I've got a 2001 260 with a 6.2 MPI Bravo 3 with about 300 hours on her. I bought it 3 years ago and found that it intermittantly surged at idle only in neutral with no load and would stop surging if it was shut down and restarted. Otherwise it ran perfectly  The problem became more frequent so I brought it to my marina mechanic to be analyzed on their computer. They said that all sensors tested fine and they would continue to try to find the problem and check with the regional Merc techs. I called frequently to check and they said they were looking into it. I finally sat down with them to ask what was the problem. I was shocked to find that they had replaced the fuel filter, plugs, coil, distrubutor cap, rotor, ignition module, IAC valve, throttle position indicator, MAP sensor and had found no improvement. They had, however, run the bill up to over $3000, including 27 hours labor! I told them that the symptoms that I described didn't indicate a mechanical problem, as they could be reset with a restart.  They said that in hindsight, that appeared to be the case. They said that it could be the ECU (computer) but they hate to buy one for several thousand dollars as it could not be returned..  We asked what the Merc tech said and they hadn't contacted them yet.  When we asked them to do so they found that our ECU had a serial number matching a batch that needed to be reprogrammed. Relief finally. They sent it in, and my reprogrammed ECU changed idle behavior but didn't correct it completely. The tech sent them another ECU to try and initially it seemed to solve the issue, but sadly, the surging returned on the next test run.  It is now mid November, almost 3 months after we initially brought the boat to them for a minor idle problem, and they had to winterize the boat for almost 500 dollars since it gets cold here in Minnesota. They now think that the problem could stem from a poor connection in the wiring harness, causing poor communication between the sensors, controllers, and the ECU.  I haven't got an updated estimate including the ECU testing but this is looking like a bottomless pit with no end in sight.  Does anyone have any ideas about the cause of my idle surge, and does anyone have any advice on how to negotiate my way out of this major bill that has not yet fixed my problem.

based on what has already been done I would start to suspect a bad or failing ECT (engine coolant temperature) sensor......this sensor tells the ECM when the engine is at normal operating temp so the ECM can start controlling things like ignition timing and fuel flow based on input from the various sensors.....the ECT is considered one of the most important sensors...if it is sending false or incorrect signals to the ECM all sorts of bad things can happen.....

you can unplug the ECT connector when the engine is surging as a test....this will cause the ECM to assume the engine is cold and will supply more fuel to the engine because it thinks it is in 'warm -up' mode....if the surging stops you have found your problem....

a failing sensor can test good but can intermittently cause issues......I chased a bad ignition control module for almost two years on a previous boat because the problem only occurred randomly....the only way I finally found it was bad is when it finally died completely.....it would test OK but fail randomly.....a new one cost several hundred $$$ so I did not want to buy one if not needed.....

as far as paying the bill did you give them permission to change all of these items?.....if so you don't have much of a case....however I am surprised that a competent marine mechanic would ever recommend throwing parts at a problem until it eventually gets fixed....I might would negotiate with the dealer and offer to pay 1/2 of the bill and they pay the other half....I question some of the parts they changed because i don't see how some of them could in any way affect the engine idle......

cliff

Does anyone have any ideas about the cause of my idle surge, and does anyone have any advice on how to negotiate my way out of this major bill that has not yet fixed my problem.

Click to expand...

Cliff is probably onto something.

As far as the bill, did you authorize the repairs? If not and they ran up a $3000 bill replacing parts and guessing at what was wrong it's time for a chat. I ALWAYS make it clear that I need to authorize any repairs beyond what we initially discuss when I drop off the boat/car etc and I get a quote for that initial repair. If you just asked them to fix it and call you when it was done that's different, you gave them a blank check and an open ended repair order. If they have your boat in their shop, it's tough to get it without settling the bill.

1. No one fixes anything at our marina without an estimate. While they certainly need time to do the diagnostics (you pay for that) and they can certainly find more issues once the patient is open, they always call or email in advance.

2. Great advice on the ECT sensor, also consider crank sensor. We chased a miss that caused a surging a the crank sensor was the problem...it tested fine as well.

I have the same symptoms on my 1999 7.4 Horizons. Both - but port seems to be more often. But the problem is still quite intermittent and conveniently seems to only happen when I am in the worst possible docking situations... seriously though, it is annoying but I haven't had the NEED to address it yet because it's still pretty infrequent. So I like the ECT sensor idea. I will keep my eye on this post.

Not certain about your engines, but cars have an idle position sensor, that will cause idle to fluctuate.

Dane174

On 2001 454 motors there is Cool fuel pump/cooler/tiny filter screen. On that fuel cooler assembly there is a fuel pressure regulator. I had the same surging issue you describe. Check your fuel pump for this regulator,(small diaghram/aluminum housing with a vacuume line on it) I am not sure if the small block motors are the same but its worth a look at the pump. I too tried all the above like you, and the regulator replacement cured it.

Thanks for the advice on ECT but I noticed that they had replaced that also. I tried to look up if a 2001 6.2 liter has a crank position sensor and it may not. Just spoke with the shop and they said that the mechanic just did a sea run, switching out the two ECU's and taking run readings on his diagnostic computer. The behavior was the same but no more sign of running rich as before. I asked if they had any other ideas and they said the Merc tech suggested an engine tear down which they knew made no sense, and their idea to replace the wiring harness ie. thousands more dollars. I asked if they could test for bad connections in the harness for which a new connector could be spliced in and he said that that is what the mechanic is now doing. I believe that I may have to cut my losses and take the boat back as I have been living with this intermittent surge since I have owned it and it only impacts docking which I can correct for a while just by shut down and restart. Good advice also from you guys on demanding contact be made if the bill starts to go past a certain point. I could not specify only what parts to test as the idle condition could have many causes. At least I know what isn't causing the issue. I imagine that the shop would never admit that they can't fix it as that would leave me in a better position to negotiate the bill down. Winter storm tonight in Duluth so they will put it inside till we can come to a resolution tomorrow. This sucks

Sublimetime said:

Dane174

On 2001 454 motors there is Cool fuel pump/cooler/tiny filter screen. On that fuel cooler assembly there is a fuel pressure regulator. I had the same surging issue you describe. Check your fuel pump for this regulator,(small diaghram/aluminum housing with a vacuume line on it) I am not sure if the small block motors are the same but its worth a look at the pump. I too tried all the above like you, and the regulator replacement cured it.

Click to expand...

Sublime, does this look like the part you are referring to? (FILTER, Fuel Pressure Regulator) - #16 on the diagram is the filter, #15 is the whole regulator.

http://www.jamestowndistributors.com/userportal/show_product.do?pid=83469

its not unheard of to get new parts and they be bad.....I know they changed the IAC valve but I'm wondering if the new one could have been bad also.....be sure the seat in the throttle body for the plunger head of the IAC valve is clean...if the plunger head does not seal the opening in the throttle body properly that will allow unwanted air into the TB and play havoc with the idle....this is a long shot but it happened to me....ensure the head of the plunger of the new IAC valve is EXACTLY the same size and shape as the original one....I ordered a new IAC valve once (not OEM) and it looked exactly like the old one but had a different shape head on the plunger.....I installed it anyway but the idle would surge because the different head was not sealing the TB opening properly.....

you can unplug the IAC electrical connector when the idle is surging to test the IAC valve....if the idle smooth's out when the connector is unplugged the IAC valve is bad....

cliff

Interesting I have the same issue on my SB engine. I have found that after warms up and I will shut it down it goes away.

CliffA said:

its not unheard of to get new parts and they be bad.....I know they changed the IAC valve but I'm wondering if the new one could have been bad also.....be sure the seat in the throttle body for the plunger head of the IAC valve is clean...if the plunger head does not seal the opening in the throttle body properly that will allow unwanted air into the TB and play havoc with the idle....this is a long shot but it happened to me....ensure the head of the plunger of the new IAC valve is EXACTLY the same size and shape as the original one....I ordered a new IAC valve once (not OEM) and it looked exactly like the old one but had a different shape head on the plunger.....I installed it anyway but the idle would surge because the different head was not sealing the TB opening properly.....

you can unplug the IAC electrical connector when the idle is surging to test the IAC valve....if the idle smooth's out when the connector is unplugged the IAC valve is bad....

cliff

Click to expand...

Cliff - Not necessarily; the IAC may be "hunting" due to ECM commanding it to do just that based upon other inputs. So, consequently, if unplugging the IAC stops the surging it simply reveals the issue is caused by the engine's fuel and control system; not necessarily a bad IAC.

YES That is the pressure regulator # 15 in Jamestown exploded view. Vacuum controlled, after three years of changing and testing and swapping parts from port to starboard motors to see if surging would move from side to side with part swapping ,this was the last thing changed before cure.

On another note- when you install a new IAC motor/valve you should recalibrate the system according to the shop manual by following a turn key on without starting then cycling it on and off for a specific amount of time in between. CHECK MERCUISER SHOP MANUAL>

ttmott said:

Cliff - Not necessarily; the IAC may be "hunting" due to ECM commanding it to do just that based upon other inputs. So, consequently, if unplugging the IAC stops the surging it simply reveals the issue is caused by the engine's fuel and control system; not necessarily a bad IAC.

Click to expand...

good point.....thanks....

cliff