350 mag mpi surging under load

[QBhoy]

Defo get the iridium plugs and rotor and take it from there. When you have electrical related faults like this, it would be crazy not to have what is specified for the engine....I'm sure the good folk at Mercruiser specify iridium for a reason.
Failing this, still thinking TPS....and to answer your question about would it not show a fault....mmm maybe intermittently, but not a given on these bloody engines. Mind of their own with this kind of thing. I can testify to this.

 

[QBhoy]

Oh....and if the new cap you have as new LAST year and before winter layup....worth replacing too. They don't like cold and damp sutained and shouldn't be taken as "good" anytime other than after just replacing....silly sounding, but true.

 

[silver_power]

The cap was changed this year sometime during August and the engine since then has been operated for about 4 hours.

 

[Fun Times]

The cap was changed this year sometime during August and the engine since then has been operated for about 4 hours.

But did the surging seem to happen before or after the new cap?

If you believe it's somewhat better then you may be headed in the right direction with new plugs and rotor plus a proper cap.

^^ Again with knowing that new info, has the distributor shaft assembly been moved (turned) at all before or even after the surging was noticed? Didn't you ask about setting "engine timing" one time before silver_power?

Though sometimes the contact terminal materials of distributor caps can change (mostly between brass and aluminum), these caps have a tendency to cross arc internally causing misfire type issues. It supposed to be less of an issue with the brass design though. But it's still imperative to have this style distributor assembly phased in correctly or you could still have an internal arc issue going on.

Service dealers now have a few ways to properly phase in the distributor settings. One by watching the CAM angle via live data on later model scan tools as you adjust. Not sure if your scan tool offers that option Silver_power? http://forum.chaparralboats.com/index.php?/topic/18294-flat-distributer-caps/

The next way is would be by following the steps mentioned in posts 15 (the quote by muc), 16 and the link in 17 by AD.... if you can get the link to work...I can't...Anyone here able to post the images of page 15 - 20??? http://forums.iboats.com/forum/engi...i-wrong-ecm-on-engine?p=10221970#post10221970

Though it's from Volvo Penta, here is the original way to phase in this distributor design as it helps explain better than most other info from Mercruiser, http://forums.iboats.com/filedata/fetch?id=7001823

If the TPS voltage or percentage was fluctuating just slightly due to say a failing, loose, corroded sensor/wire, etc., then chances are the ECM may not necessarily set a fault code due to the movement might not be falling out a set voltage range which should be about .5 to 5.0 volts per se. If the volts went to say 0- or 6.0+ volts then it's supposed to set a fault.
If the ECM "thought" the TPS was moving say between 2 or 4 % (or about 1 volt) then yes the engine may want to try and change all the other ECM mappings making the engine surge a bit even though the throttle handle really isn't moving allowing more air into the engine.

But using the live data on the scan tool or using a multimeter, you should be able to see if the voltage is actually changing or not.

 

[tpenfield]

Just having re-read through this thread, I am wondering about the following. . .

1) I did not see any description of how the problem came on. There is mention of the 'normal' WOT RPM and now the lower WOT RPM, but nothing about when the problem started to happen.. Suddenly? Gradually? Anything of note around the time it started to happen?

2) It seems that there has not been any diagnostics (Rinda computer program or scanner) done while the engine/boat is running under load and experiencing the surging. So, everything done seems to be guesswork in the absence of live data from the engine.

3) Not sure how much money has been spent on this problem so far, but I would guess it to be well over the cost of buying the full Rinda Diacom or having a mechanic go out with such and run a recording of the engine. Lots of folks balk at the price of the Diacom software, but based on all the 'part replacing and swapping' that we tend to see in these situations, it may be the low-cost alternative.

 

[alldodge]

.Anyone here able to post the images of page 15 - 20???

I don't understand posting which images?

Here is the pdf file in post 17

 

[Fun Times]

Thanks AD!!!
I'm unable to open any of the bulletins from crowleymarine.com as it keeps saying

It appears you don't have a PDF plugin for this browser.

So there's no link for me to open.:noidea:

So my first thought while making that post was maybe someone could screenshot pages 15 - 20 and post them as I wanted to see what they said not knowing (thinking) you could post an attach file like that...Your idea was much better. Thanks again for the help.:thumb:

 

[alldodge]

The actual screen shots were pretty small in jpg so chopped it up

 

[alldodge]

Last set

 

[QBhoy]

My MPI engines run well without iridium plugs :noidea: I just use the AC/Delco marine plugs.

Iridium . . . what is that anyway :noidea: . . . sounds like kryptonite

Just thought I'd mention, for the sake of any future reference to this post.....
The older generation 7.4 MPI does not require iridium plugs. This is stated as such in manual. This is a much older tech engine.

The newer generation MPI are different engines all together. They DO require iridium and are stated as such in the manual.

 

[silver_power]

The mechanic has checked the timing of the engine and it is correct and he found it ok. I believe that my problem is the plugs as these I now use are used, non iridium , and for sure I do not know if they are gapped. As here in Greece are not always available the mercruiser parts I still wait for a new rotor and new iridium plugs. The mechanic has came on boat for test with the scan tool twice to check and test sensor (swap etc) and after all he tested (everything mentioned above) he waits thet new parts to come and teSt them as well. I will post results as soon as I test them. The good is that the boat is better now with the used rotor and the wire I changed.

 

[tpenfield]

Just thought I'd mention, for the sake of any future reference to this post.....
The older generation 7.4 MPI does not require iridium plugs. This is stated as such in manual. This is a much older tech engine.

The newer generation MPI are different engines all together. They DO require iridium and are stated as such in the manual.

QBhoy I am trying to look that up in the manual, but I am not sure I have the correct one for the OP's engine. Which manual is it and where does it state the requirement for iridium plugs? I checked manual #31 and that calls for AC Platinum plugs '41-932'

EDIT - I think I found it . . . calls it out on the parts list (not the manual :noidea: ) originally called for the platinum, (NGK 5599) now calls out the Iridium (NGKITR4A15).

 

[QBhoy]

QBhoy I am trying to look that up in the manual, but I am not sure I have the correct one for the OP's engine. Which manual is it and where does it state the requirement for iridium plugs? I checked manual #31 and that calls for AC Platinum plugs '41-932'

EDIT - I think I found it . . . calls it out on the parts list (not the manual :noidea: ) originally called for the platinum, (NGK 5599) now calls out the Iridium (NGKITR4A15).

Hi TP

Just picking these up now...been crazy at work.
Glad you found the details. You are spot on about the NGK irridiums. I think at one point the AC Delco 993 was recommended too....change their mind all the time !

 

[silver_power]

Well finally spark plugs arrived and changed them with new ngk itr15a. The boat had not been operated for the last 20 days.At first ride everything was perfect for a 10 minutes ride and I was happy for finding boat's surging problem. After a 45 minutes break, I started again the boat. I got it on plane and it started again small surging problems which getting worse minute by minute. I made a 30 min ride for several tests. The result is that the boat surges from plane till 3400rpm. Above 3400rpm things are a lot better and at 3800-4000rpm it rides smoothly till about wot @5000rpm. Something that I noticed is while ride at about 3000rpm, surges from 2900 to 3100rpm and smartcraft reads 2,8mpg . At 3800rpm with no surge smartcraft reads 3mpg! So the problem exists below 3600-3800 and not above that. Something else I noticed is that while I was docked I got the throttle only (without gear)at 1100rpm but the engine could not handle steady rpm. It went from 1100 to 1300 and back and again for about 3-4 times and after that it handled 1100rpm steady. That happened for the first time today. Generally The idle is fine (600 with gear, 550-570 without gear). No problems woth idling. I am starting to think that something is wrong with the tps. Probably i must tell the mechanic to come again with the scan tool to test if with steady throttle the tps has a steady signal or not. Do you think that the symptoms I describe might be due to bad tps?

 

[silver_power]

Update:
I still can not find the problem of the engine. It still surges from plane till a aprox 3500rpm. Then the surging stops. Distributor cap new, rotor new, plugs new, wires new, mapt sensor new. The tps sensor is ok since I tested it real time. It gives steady voltage while surging. I measured also the output voltage of the tps and it raises totaly harmonic from idle to wot. 0,665v@idle - 4,65@wot and 5,0v to the sensor. I also checked the grounding of the engine and everything seems tight and clear.
Something I noticed is that the fuel pump is louder than it was previous years. Anyway , something i just remembered ,the mechanic told me that at last service he changed the fuel pressure regulator as the previous was corroded. Since then I have the surging problems. ...I do not know why but I want to check the fuel pressure while it surges and I am saying that because 1) my mechanic told me he checked the pressure but I am sure that he did not checked it while underway and 2)sometimes while surging I hear some noises from the engine like backfire just like my old carb bmw made when she had problem with the carbutator. He says that since it is ok @wot the problem does not come from fuel. Partly I agree but I just want to see the pressure with the gauge while surges as I did with the tps sensor. Is it possible that he put by mistake a 30psi fpr instead of 43psi? Would the engine reached 4900-5000rpm hitting 57mph@gps and run like champ having fuel problems , i doubt it but what else should I check?? Maybe a vacum leak?...but I do not having problems at idle or low rpm. The problem occurs since getting boat on plane and below 3500!. What else should I check. Ps: still no fault codes at scan tool.

 

[alldodge]

Check fuel pressure while running

something i just remembered ,the mechanic told me that at last service he changed the fuel pressure regulator as the previous was corroded. Since then I have the surging

Is it possible that he put by mistake a 30psi fpr instead of 43psi?

A new piece of the puzzle

 

[silver_power]

Is possible to a non turbo engine (things there a different) to have problems with the pump or fpr only to mid range and not at wot?if he put 30psi fpr (pre 2001 engines) and not 43psi (newer mpi's) I think that the engine would starved at high rpm. Is it correct?
In any case I will borrow a gauge to test the fuel pressure by myself and @idle with the vacum hose untouched I will be sure if the fpr is correct and what rating it is.
Believe me I would prefer to had problems ?t wot and not at cruising range......

 

[alldodge]

Engine could be lean, could be rich and right now have limited info. Normally if the motor should be running at 43 and is only running at 30 psi there should be an issue and I would think it would also be at WOT. That said, I for one need to know what the parameters are and not what we think they are.

 

[silver_power]

This week I will take the boat for winter storage but before that I will check the fuel pressure with a gauge. Is there anything else that I should check to refer you to tell an opinion?

 

[alldodge]

Currently I'm leaning toward fuel pressure, if fuel pressure turns out to be steady, and 5V circuit is steady then I would have the ECM looked at