MPI, questions

[Tidbart]

97 502 MPI with MEFI 1 newly rebuilt in 08, during rebuild, I replaced the IAC and TPS, went through the whole fuel system, and just about everything else. The only sensor I didn't change was the MAP. Replaced all vacuum lines, inspected every inch of wire and connectors on the harness and retaped them.

Problems:
Prior to rebuild: I had a minor surge issue, in neutral at around 1100 rpms, it would start to hunt: varying from about 900-1200 rpms. Higher or lower rpms weren't an issue and I never noticed it in gear.
Always started right up.
After rebuild: I replaced TPS and IAC. Minor surge issue still remains.
No I have been getting other issues. When I back off the throttle, quickly, sometimes the engine will stall. Also, sometimes when I go to restart it, I have to give it some throttle in order to open the plates to allow more air in to get it to fire. Also, along with these occasional issues, I am getting and occasional rough, low, idle, and stalling while in neutral.

Buizilla suggested, just based on the low-idle, stalling upon slowing, issue, to adjust the idle screws on the throttle body primary, but I think I have more issues than that. The MPI should not ever have to change the idle screw setting. They are fixed at the factory and cemented in place (and a bitch to get to without removing the throttle body). The ECM should adjust and control the idle as needed and I don't want to treat the sympton.
I did remove the throttle body yesterday and inspected the linkages and can see that the idle primary idle screw is exactly set where it should be. Any movement on the linkage, at all, will start to open the plate.

I hooked up a scan tool to my MEFI 1 to read outputs and sensors, NO ERROR codes, understandable.

I am getting some funky feed back from the IAC motor.

IAC Postion
2 days ago:
Eng off. 127
Idle 137
900 rpm 133
1250 rpm 136 (Looks like it stuck open??)

Yesterday
Eng off. 130+
Idle 130+
900 rpm Descended to 0
1250 rpm Descended to 0

The 0 position should be all the way seated in the bypass of throttle body.
I guess my question is: Am I reading...
1) info the ECM is giving to the IAC, whether it is responding or not.
2) the actual position of the IAC in real-time.

Sounds to me like the (13 month old) IAC is sticking on occasion.

Other reading that concern me:
TPS Volts at 0/600/900/1250 RPMS are 0.62/0.66/0.78/0.82 The last being slightly out of range (0.4-0.8 volts)
Throttle percent for the same readings was 0/0/3/4. Thought they would be higher.

MAP Pressure at 0/600/900/1250 RPMS...in Hg........29.4/16.6/13.7/10.2
MAP Volts for the same are.........4.9/2.4/1.92/1.35 Are these going in the right direction???
Also,, yesterday, I disconnected the MAP, started the boat, it ran a little rough at idle, but when I started to give it gas, it started to surge wildly from say about 800- 2000 rpms, shut it down. It did correctly set an error code.

Sorry for the babbling post.

Any thoughts? :bonk: Replace the almost new TPS, IAC, and old MAP?? Checked base timing, right on the money. Vacuum leak? not likely.

Random guesses from those not familiar with MPIs, not welcome.:wink::)

Bob

[mrfixxall]

That system is based off of the coolent temp sensor, it its out of range it will throw the readings off,is the engine running at normal operating temps? also if the tps shows any% at idle it will keep the iac closed because the pcm is thinking the throttle blades are still open and wont open the iac to let it idle..check the tps for throttle % if its more then 1% at idle it may not be letting the iac open..check for oil or sludge in the throttle blades,that will give you a irattic idle,also check for vacume leaks..

[A2VeeDub]

A can of either works wonders for finding vacuum leaks. Just be careful where you spray it, if you like your hair. mrfixxall covered the most important stuff. Could be a MAF issue, but check the other stuff first.

[Tidbart]

This may help. Sunday data is thorough. Monday, I was looking at the MAP and IAC data.

Throttle Body is spotless. No MAF, only MAP, could be typo.:boggled:

Temps are normal and working.

Will check vacuum tonight.

B

[mrfixxall]

Try cleaning the map sensor,looks to me like the baro voltage is not changing,if i recall correctly its soppose to read some where 1.28 volts at idle and your is reading 4.93 volts alomst like the hey is on and the engine is not running..also i noticed your battery voltage is 12.3-should be 12.6..mabe a weak battery and the alternator is spiking the ecm..also make sure theirs enough vacume at the map sensor port...when you rebuilt the engine did you use the same cam?

[A2VeeDub]

No MAF, only MAP, could be typo.:boggled:


Thinking one thing, wrote another.

I am not sure what the number on the IAC represents. % open or something? The Sunday run is curious as mrfixxall said it should close as soon as the TPI reads open. The monday looks right.

[Tidbart]

Try cleaning the map sensor,looks to me like the baro voltage is not changing,if i recall correctly its soppose to read some where 1.28 volts at idle and your is reading 4.93 volts alomst like the hey is on and the engine is not running..also i noticed your battery voltage is 12.3-should be 12.6..mabe a weak battery and the alternator is spiking the ecm..also make sure theirs enough vacume at the map sensor port...when you rebuilt the engine did you use the same cam?

The first column is "Boat off, key on, take readings on scan tool". Second is the idle numbers.

I can also switch over to the other Battery and retake the readings.

Rebuild was to stock specs. Same cam, same everything that wasn't normally wearable. I don't know if you remember, but I ingested some water in May of 08 due to rotted intake manifold. Saved the engine, rebuilt to stock. All external replacements were Merc, all internals GM.

What it amounts to is this is pretty much a brand new Merc 502 Mag.
If I remember correctly, the only parts not replaced were the distributor and module, ignition coil,and the MAP sensor. The only parts "rebuilt" were the injectors. Had Jim(Buiz), test and clean those.

B

[Tidbart]

Thinking one thing, wrote another.

I am not sure what the number on the IAC represents. % open or something? The Sunday run is curious as mrfixxall said it should close as soon as the TPI reads open. The monday looks right.


IAC numbers represent counts, or step. A high number should be, pintle all the way back, allowing air to flow around the TB shutters. Then as you move the throttle away from idle the IAC steps the pintle in to 0, to block the bypass tube.
If you look at the first day's numbers, the position stayed high even though I upped the RPMs. Not right. Second day appeared to work properly, IAC went to 0.
This would lead me think that IAC motor isn't always working they way it should, but I still believe I have other issues. TPS, MAP or vacuum. Although, knowing how meticulous I was with reassemble, vacuum should not be an issue, although I will still check.

Performance is still there, hit 70, alright 69.8 (gps), on Saturday with high humidity and in the 90's.:yes::) It's below 1500 rpm that bugging me.

B

[Tidbart]

Just looked through my history of invoices. It appears that in a moment of weakness, or ignorance, or cheapness,:bonk: I ordered a Sierra IAC motor. This was months before the rebuild. Judging from my numbers, I think it may be sticking, so I just ordered one and will replace it upon arrival, then see what happens and go from there.
In the mean time, I will keep looking for other issues.

B

[BUIZILLA]

In the mean time, I will keep looking for other issues. sounds like my doctor's conversation this morning... :bonk:

[Tidbart]

Quit buying the sierra sh1t...

I did, right after I purchased that IAC motor:bonk:

[VetteLT193]

Usually IAC stays fully open when running. this way if you drop the throttle down to dead idle suddenly it won't stall the engine.

I wonder if the out of spec TPS causes the computer to drop the IAC to 0?

if the TPS is not smooth, or giving incorrect feedback, it could do the same thing @ idle.

[Tidbart]

Usually IAC stays fully open when running. this way if you drop the throttle down to dead idle suddenly it won't stall the engine.

I wonder if the out of spec TPS causes the computer to drop the IAC to 0?

if the TPS is not smooth, or giving incorrect feedback, it could do the same thing @ idle.

I ordered a new Merc IAC, will try that first, see what happens and go from there. Should be here in a few days, I hope.

TPS in new Merc last July, would hope it is not an issue. I can see the volts throughout the range with the engine off.

B

[VetteLT193]

I ordered a new Merc IAC, will try that first, see what happens and go from there. Should be here in a few days, I hope.

TPS in new Merc last July, would hope it is not an issue. I can see the volts throughout the range with the engine off.

B

That would be an easy fix at least, hopefully that does it. :wink:

[silverghost]

This is exactly why I am not a big fan of electronic fuel injection on a boat engine!
FI engines are great and give you super power & fuel economy, as well as low exhaust emissions when they are working well~
But all those sensors do not hold-up well in the damp & salty marine environment!
Long-term winter storage does not help this issue.
A friend has an Indmar Cadillac Northstar powered Mastercraft skiboat.( built for one year only!)
It was a high-cost experiment!
He is always tearing his hair out with FI sensor failure issues!
I know of no marine engine builder still using the Cadillac Northstar engine system?!
Side Note:
Indmar is now using catalytic converters built into many of their Costly $$ marine manifolds!
Many other marine Inboard, ouboard and PWC engines are now also gowing "Green" with electronic Fuel Injection!
The future is here ~ thanks to Uncle Sam & the EPA !
I will stick to my big carbs & simple ignition systems as long as the EPA & government will allow me to do so!
California has already banned these "old School" fuel systems & ignition systems!
I have been where you are and I know your frustration!
Good Luck !
Brad Hunter

[Pismo]

This is exactly why I am not a big fan of electronic fuel injection on a boat engine!
FI engines are great and give you super power & fuel economy, as well as low exhaust emissions when they are working well~
But all those sensors do not hold-up well in the damp & salty marine environment!
Long-term winter storage does not help this issue.
A friend has an Indmar Cadillac Northstar powered Mastercraft skiboat.( built for one year only!)
It was a high-cost experiment!
He is always tearing his hair out with FI sensor failure issues!
I know of no marine engine builder still using the Cadillac Northstar engine system?!
Side Note:
Indmar is now using catalytic converters built into many of their Costly $$ marine manifolds!
Many other marine Inboard, ouboard and PWC engines are now also gowing "Green" with electronic Fuel Injection!
The future is here ~ thanks to Uncle Sam & the EPA !
I will stick to my big carbs & simple ignition systems as long as the EPA & government will allow me to do so!
California has already banned these "old School" fuel systems & ignition systems!
I have been where you are and I know your frustration!
Good Luck !
Brad Hunter

Here, Here..Get a carb and a Mallory distributor and go boating....30 second repairs.

[Tidbart]

This is exactly why I am not a big fan of electronic fuel injection on a boat engine!
FI engines are great and give you super power & fuel economy, as well as low exhaust emissions when they are working well~
But all those sensors do not hold-up well in the damp & salty marine environment!
Long-term winter storage does not help this issue.
A friend has an Indmar Cadillac Northstar powered Mastercraft skiboat.( built for one year only!)
It was a high-cost experiment!
He is always tearing his hair out with FI sensor failure issues!
I know of no marine engine builder still using the Cadillac Northstar engine system?!
Side Note:
Indmar is now using catalytic converters built into many of their Costly $$ marine manifolds!
Many other marine Inboard, ouboard and PWC engines are now also gowing "Green" with electronic Fuel Injection!
The future is here ~ thanks to Uncle Sam & the EPA !
I will stick to my big carbs & simple ignition systems as long as the EPA & government will allow me to do so!
California has already banned these "old School" fuel systems & ignition systems!
I have been where you are and I know your frustration!
Good Luck !
Brad Hunter


Here, Here..Get a carb and a Mallory distributor and go boating....30 second repairs.

This is not all that big a deal. I have an issue just like everyone else will have an issue. I actually enjoy the challenge of researching, analyzing, finding, and repairing a problem. Yes, I could band-aid it and treat the symptom or just go replace everything, but, in my mind, that would accomplish nothing.
I actually love the Merc MPI system and the benefits of it, as you stated earlier. We would both have to agree that most FI marine engines are not like the Cadillac example and its not to say simpler systems don't have their issues.
I will get this figured out with the help and input of guys like you who, but let's not turn this into a carb vs FI thread.:wink::bonk:

Bob

[VetteLT193]

This is exactly why I am not a big fan of electronic fuel injection on a boat engine!
FI engines are great and give you super power & fuel economy, as well as low exhaust emissions when they are working well~
But all those sensors do not hold-up well in the damp & salty marine environment!
Long-term winter storage does not help this issue.
A friend has an Indmar Cadillac Northstar powered Mastercraft skiboat.( built for one year only!)
It was a high-cost experiment!
He is always tearing his hair out with FI sensor failure issues!
I know of no marine engine builder still using the Cadillac Northstar engine system?!
Side Note:
Indmar is now using catalytic converters built into many of their Costly $$ marine manifolds!
Many other marine Inboard, ouboard and PWC engines are now also gowing "Green" with electronic Fuel Injection!
The future is here ~ thanks to Uncle Sam & the EPA !
I will stick to my big carbs & simple ignition systems as long as the EPA & government will allow me to do so!
California has already banned these "old School" fuel systems & ignition systems!
I have been where you are and I know your frustration!
Good Luck !
Brad Hunter

The Merc system is a far more simple setup than the Northstar system. The engines themselves are far more simple too... Northstar is really the next generation LT5 (original Corvette ZR1). they were the bleeding edge of technology and have a TON of sensors. Merc tends to trail in technology on purpose to keep thing smore simple and reliable.

[VetteLT193]

Hey Bob... did you get it installed yet? Just curious

[Tidbart]

Hey Bob

Been busy. Lightning took out washing machine, a/c, and pool pump. Spent a few days working on that stuff. Got IAC on Tuesday but couldn't put it in because I had rotor cuff surgery on Wed.

May be a couple of weeks before I can get back to it. Typing is tough, too. LOL

Bob

[VetteLT193]

Hey Bob
Been busy. Lightning took out washing machine, a/c, and pool pump. Spent a few days working on that stuff. Got IAC on Tuesday but couldn't put it in because I had rotor cuff surgery on Wed.
May be a couple of weeks before I can get back to it. Typing is tough, too. LOL
Bob

OOF. hang in there man. Hopefully you'll heal quick :crossfing: