Mercruiser EFI Stalling and Surging Symptoms at Idle

[rossi45]

Well, I'm prety much at my wits end and the bottom of my wallet. I've been having an issue with my engine where is surges and stalls at idle.

Engine specs:

• 5.7 Mercruiser EFI Stern Drive, Bravo III.
• Engine serial number 0M033393
• Throttle body injection

The symptons are:

• When I start the engine from cold, I allow the engine to warm up at idle. This goes smoothly everytime. When I rev up the engine, either in neutral or under load, the engine works fine. It's when I bring the engine back into idle, the RPM's will drop and the engine will almost stall, and then catch itself and surge up to about 1500-2000 RPM's and then will drop back again to almost stall and then surge again and so on and so forth until it actually stalls out or I turn off the engine.

Diagnostic steps:

• Myself
  • Replaced fuel filter and checked fuel for particulates. Fuel was clean. No water. Topped up engine with new fuel anyways.
  • Replaced Idle Air Control. Made sure to re-learned IAC a few times to make sure it was adjusting properly. I even pulled IAC and turned the ignition to start to make sure the pintal was operating. IAC is not the problem. No change in symptoms.

• Took boat to mechanic 3 weeks ago. He went to working:
  • Plugged up to the ECU to look for fault codes. None exist. All diagnostic outputs say engines is working fine.
  • Rechecked IAC using computer. All works fine. No change in symptoms.
  • Checked plugs and wires. Noticed one plug had a crack. Replaced all plugs on engine. No change in symptoms.
  • Checked for Vacuum leak. None exist.
  • Checked fuel pressure. Pressue within 1 lb of manufacturers spec. No issue there
  • Replaced fuel regulator. No change in symtoms.
  • Replaced fuel pump. No change in symptoms.
  • Replaced map sensor. No change in symptoms.
  • Checked for exhaust leak. None found.
  • Checked timing. Timing was bang on.
  • Mechanic sent ECU files over to Mercruiser for diagnosis. They couldn't find anything wrong, but suggested it might be injectors.
  • Checked injectors at throttle body. They work fine. On chance, he had a second throttle body in the shop. He replaced mine with that one, including injectors. No change in symptoms.
  • Mechanic also says he can trick the engine into thinking it's cold (even after warmup....unplugs a sensor I think, not sure which one). When he does this, the symptoms go away. So the issue seems to be related to warm engine.

Both my mechanic and I are both banging our heads. Last ditch effort is to have the ECU reflashed today or early next week. Beyond that, the mechanic and Mercruiser have no other ideas how to resolve this. This is why I am here. If anyone can throw out some other possibilites, I am all ears and open to suggestions.

 

[alldodge]

Mechanic also says he can trick the engine into thinking it's cold (even after warmup....unplugs a sensor I think, not sure which one). When he does this, the symptoms go away. So the issue seems to be related to warm engine.

You have been dealing with a puzzling issue. The may to trick the motor to thinking it is cold is to unplug the ECT (engine coolant temp) sensor. The ECT tells the computer it's cold and the engine needs more fuel. More fuel increases the pulse width for the injector(s).

The MAP sensor if defective would cause the engine to run rich, it should be a 1 BAR sensor. Don't think it's that.

I would suggest checking your fuel pressure, it should be 30 psi at 1800 rpm. There is also a vacuum line which goes between the fuel pressure regulator and the intake manifold. Removing the line will increase the fuel pressure.

Would also suggest connecting a vacuum gauge on the vacuum line to see if your pulling 15 to 21 in Hg at idle

Note to self: motor is a 1998 model

 

[rossi45]

You have been dealing with a puzzling issue. The may to trick the motor to thinking it is cold is to unplug the ECT (engine coolant temp) sensor. The ECT tells the computer it's cold and the engine needs more fuel. More fuel increases the pulse width for the injector(s).

Thanks AllDodge for chiming in. Thanks for pointing out the ECT. That's exactly what he did. The mechanic and I spoke about it today. He explained to me exactly what you just mentioned, about how it tricks the engine into needing more fuel.

As I mentioned, he replaced the Map sensor, so I don't believe that's the isssue either. I'm still open to the idea that, somewhere along the way, even a new part he installed could have been defectiive. Very highly unlikely but a consideration none-the-less.

The mechanic is so frustrated, he said he went over fuel pressue and any vacuum leak issues a few times. He can't find a single one.

On your note to self AllDodge, the engine is a 1998 model however it is a whole new longblock installed in August of 2014. Neither the mechanic, nor Mercruiser thinks it's a block issue, however things are getting to that point....

Note to myself: Ask the mechanic if he checked pressue for 30psi at 1800 rpm's. That's a good question to ask.

 

[alldodge]

Replacing the motor in 2014 should not have an effect on the issue. After looking up your serial number it came up with a 1998 model so I needed to know that to look in manual 24 to look up the specs for yours. Put a note to self so when I come back to this thread it will remind me what manual I need to use.

Not to many mechanics I've found have a vacuum gauge, so if he has one see what he finds. Would be good to know if it is within range and steady, wavering or any kind of fluctuations. What it does can tell us a lot about what's going on

 

[rossi45]

I'll be chatting with the mechanic later today or Monday when I go out to the dealership. I'll ask if he has a vacuum gauge and that it's within the specs you mentioned from the manual. I guess I assumed that he would have the proper tools to do the job. I'll find out for sure.

This is quite a frustrating venture because the weather in the PNC/Vancouver Island is really starting to get nice!

 

[Fun Times]

Exactly as I was going to say AD.

As another test to maybe stop the surging, Pull the vacuum line off the regulator and see if the issue stops. Also inspect the vacuum line/s for any damage or clogging at all from end to end. Does the fuel pressure move up roughly 3 - 6 psi when you remove the vacuum line from the regulator? It should.

By pulling the engine coolant sensor could indicate a fuel problem at low RPM to the injectors affecting fuel pulse numbers. Using the scan tool, keep an eye on the live data settings under fuel pulse width to see if the numbers change dramatically before & during the event....Also watch the TPS volts and % value numbers.

Part two of pulling of the coolant sensor is the ECM thinks the engine is cold so in-turn the RPM will be higher....Closer to 700 RPM and the desired idle will (should) be set higher as well which could be another reason the engine won't surge when cold because the sensor values are all higher and the ECM is compensating for those readings.
When it's not surging and warmed-up, what is the normal/lowest idle RPM, desired idle and TPS values you are reading on the scan tool. Here's sort of a reference point of discussion covering desired idle, http://forums.iboats.com/forum/engin...-2l-iac-sensor

The Injectors may need to be cleaned and flow tested. Search injector cleaning online for someone in your area to possibly do it or there are how to YouTube videos online to try it yourself too.

Or the ECM may need it's fuel mapping tested and recalibrated. If so call up Bob at MEFIburn and see if he can help, http://mefiburn.com/mefiecms.asp

This engine is Engine is MERCRUISER MEFI 3 ECM with 3" RISERS 5.7L EFI BRAVO

 

[rossi45]

I'm just reading through the engine manual. and looking at the fuel system flow diagrams. Under the flow diagram for the Multiport Injection flow diagrams (for either MEFI 1, 2 or 3), I see where the vaccum line is. There's a line called "Diaphragm Rupture Line To Flame Arrestor" under the Throttle Body Injection flow diagram which is in the same place on the diagram where the vacuum line is under the Multiport flow diagrams. Are they one in the same thing, just specific to the injection setup type?

 

[alldodge]

I'm just reading through the engine manual. and looking at the fuel system flow diagrams. Under the flow diagram for the Multiport Injection flow diagrams (for either MEFI 1, 2 or 3), I see where the vaccum line is. There's a line called "Diaphragm Rupture Line To Flame Arrestor" under the Throttle Body Injection flow diagram which is in the same place on the diagram where the vacuum line is under the Multiport flow diagrams. Are they one in the same thing, just specific to the injection setup type?

Manual 24 has carbureted and EFI contained within. The line which goes from the mechanical fuel pump to the flame arrestor is used when "if" there is a rupture of the primary diaphragm of the pump. If this happens the fuel is stopped by the secondary diaphragm and fuel is dumped into the engine intake instead of the engine crankcase.

The line on a TBI or MPI is used to go to/from the regulator and dampener or intake. If the regulator does rapture a similar issue could result but in most cases no fuel enters the intake.

 

[airshot]

Has anyone checked for a blown or burned head gasket? My older non injected engine started having issues similar to yours and found a burned head gasket between two cylinders. Just a thought here.........might run a compression check to be sure.....

 

[rossi45]

Thanks for the info AllDodge. Very Valuable as usual. I'm going to see the mechanic on Monday. I will make a point of raising the vacuum test question to him.

Airshot, really good point about the gasket and compression test. My mechanic did do a compression test too. I forgot to mention that. The test turned out fine.

 

[Tail_Gunner]

A thought if tricking the ecu into thinking its a cold engine does the trick then thats where i would start. Mercrusier does have a cold start mode as stated above it's simply a bit more pulse width of fuel and is pretty illuminating. If your running lean the first thing that would happen would be detonation and then the ecu would retard the timing now things's could get messy if if your really lean and the ecu is trying to clean that up.

Replacing the fuel pressure regulator is of concern in the fact that only marine tbi uses 30lbs did he use the exact right spring automotive tbi use's 12 i think

You made a engine change do you know for a fact your old engine was a true vortec motor your very close to the transition period....aka if the old motor was not a true vortec and you installed a vortec then yes it would run lean. Is this boat new to you take nothing for granted visuliaze and inspect both engine to be sure they are apple's to apples

That cold enrichment trick is suttle in nature but glaring in performance. That is if it solves the problem

 

[rossi45]

Thanks for pitching in Tail_Gunner. To answer some of your questions. I bought the boat last spring. The previous owner is the one who put the new engine in. The marine dealership that is currently working on my boat is the same one that installed the engine. I never saw the old engine, so I can't compare, but I believe they put in the exact same engine that came out.

I also believe he replaced the fuel regulator to see if that resolved the initial problem but it didn't so the old regulator was not faulty. The mechanic has been working in conjunction with Mercruiser to trouble shoot the problem, so I guess I have to believe that they are replacing the proper parts and doing the proper tests. I'm glad you brought that up, though. Another good question to ask the mechanic to double check he's replacing the right parts.

 

[rossi45]

I spoke with the mechanic yesterday. We went over all the tests he performed and parts he replaced. We spoke about the issue where pulling the ECT seemed to have resolved the problem which, as AD mentioned earlier, means that more fuel is getting to the throttle body because the computer thinks the engine is cold after pulling the ECT. I guess this is where they want to get the computer reflashed. The thought is, that the computer is not reading out the proper volume to the injectors at running temperature. The hope is the reflash should fix it. Apparently, there is a fellow around town that does ECU programming on Mercruier ECU's. They are trying to find him to see if he can come out and do some "tweaking" on the ECU to make it spray the proper volume of fuel.

The mechanic also had to go to a course in Vancouver last week, for some Mercruiser training. He brought up the issue with my boat with his class and their instructors. This whole issue, along with all the approproiate troubleshooting the mechanic has done, had really boggled the mind of all of them. It seems like this ECU tweaking is the last pobbiel thing they can think of. I'm Still open to ideas though.

 

[Tail_Gunner]

http://forums.iboats.com/forum/engin...t-vs-auto-part

http://forums.iboats.com/forum/engi...mercruiser-efi-idle-after-warmup-quits-engine

What parts have you changed on this engine.....have you switched throttle bodys what injectors are in there now???

 

[Glastron_V210]

I had the same issue with my home built throttle body injected 305. I checked everything. I mean everything. Your check sheet looks very much like mine! I went as far as reprogramming the ecm and pulled out all the advance, and unplugged the iac and it still surged. WTFFFF????

The issue ended up being that the throttle cable was not allowing the throttle plate to fully close due to the cable being a tiny bit loose and sticky, and the original throttle return spring not being strong enough to overcome this bit of stickyness to fully close the throttle plate.


As a result the throttle position sensor gave too high an initial reading to the computer for 'idle' and also the throttle plate bing cracked acts like a vaccuum leak and the idle air control valve resets to 0 to try to control the rpm down. Next time you throttle back the throttle plate closes fully and the iac tries to open up but it's all over the place...causing the surging. Mine did this when warm only as well. Why? Not sure, but I suspect the throttle plate tolerences change a bit when everything expands??

Got a lead from this document:

http://www.gmcmidwestclassics.org/Web pages/Tuning the TBI.pdf

and this:

http://www.gmtuners.com/tech/TPS_IAC.htm


You can check the spray pattern of the injectors with a timing light BTW. It rules a lot of injector bs out.

Maybe you have the same thing.


Chay

 

[Fun Times]

By chance do you know if he has tried pulling the vacuum line off of the fuel regulator and watch the fuel pressure go up on the gauge then test the engine to see if the surging stops with the line removed. That would be like adding fuel without the ECM noticing. Pulling the ECT, the ECM will notice and automatically raise the idle RPM which if the idle RPM is to low to start with, the IAC may have a more difficult time to compensate.

Also be sure the fuel pressure isn't real bouncy.

Did you find out if the dealer install just a new long or short block of the same vintage and bolted on the all the original accessories or was it a complete turn key drop in engine?

 

[rossi45]

http://forums.iboats.com/forum/engin...t-vs-auto-part

http://forums.iboats.com/forum/engin...p-quits-engine

What parts have you changed on this engine.....have you switched throttle bodys what injectors are in there now???

Thanks for posting up my previous posts, Tail_Gunner. That's some good reference for this post. I started a new one this time, to tie everything together. As for your question about the parts, please see my initally post in this thread. It's all there.

I had the same issue with my home built throttle body injected 305. I checked everything. I mean everything. Your check sheet looks very much like mine! I went as far as reprogramming the ecm and pulled out all the advance, and unplugged the iac and it still surged. WTFFFF????

The issue ended up being that the throttle cable was not allowing the throttle plate to fully close due to the cable being a tiny bit loose and sticky, and the original throttle return spring not being strong enough to overcome this bit of stickyness to fully close the throttle plate.


As a result the throttle position sensor gave too high an initial reading to the computer for 'idle' and also the throttle plate bing cracked acts like a vaccuum leak and the idle air control valve resets to 0 to try to control the rpm down. Next time you throttle back the throttle plate closes fully and the iac tries to open up but it's all over the place...causing the surging. Mine did this when warm only as well. Why? Not sure, but I suspect the throttle plate tolerences change a bit when everything expands??

Got a lead from this document:

http://www.gmcmidwestclassics.org/We...0the%20TBI.pdf

and this:

http://www.gmtuners.com/tech/TPS_IAC.htm


You can check the spray pattern of the injectors with a timing light BTW. It rules a lot of injector bs out.

Maybe you have the same thing.


Chay

Chay, thats a really good thought! I'm sure that's not anything the mechanic would have looked at. I will definitely ask if he's checked the throttle plate and throttle cable. I'll actually give him a call this afternoon and ask.

By chance do you know if he has tried pulling the vacuum line off of the fuel regulator and watch the fuel pressure go up on the gauge then test the engine to see if the surging stops with the line removed. That would be like adding fuel without the ECM noticing. Pulling the ECT, the ECM will notice and automatically raise the idle RPM which if the idle RPM is to low to start with, the IAC may have a more difficult time to compensate.

Also be sure the fuel pressure isn't real bouncy.

Did you find out if the dealer install just a new long or short block of the same vintage and bolted on the all the original accessories or was it a complete turn key drop in engine?

Fun TImes, when I call this afternoon, I'm going to ask the mechanic about what you mentioned too. I'll see what he has to say to that. Thanks for the tip.

Also, as for your question about the engine. A full long block was replaced (I have the receipt) and all original accessories were taken off the old engine and bolted onto the new one, so not a turn key crate engine.


Thanks everyone for throwing their ideas out there!! This has been a very trying experience of patience and sanity!

 

[Tail_Gunner]

All things taken into consideration,a non vortex motor was replaced with a vortec motor about a 40 hp increase and answers the cold enrichment puzzle. Now with that said the intake manifolds would be different a non vortec manifold would not bolt on to true vortec heads peace out.

 

[oddev]

Any news here? I have the same issue on my 4.3 EFI.

 

[Chill 65]

Have you fixed it?
I have similar problem on start up, sometimes, with a GM 5.0 L with Throttle Body Injection.