1996 Mercruiser GenII 4.3 TBI Idle problem

[Quahog]

Hello All,
First here's the symptoms that I'm experiencing, The engine on cold start and sometimes once it has settled will Idle perfectly without skipping a beat, Once I return the engine to idle it will roll or hunt continuously from 500 to 1800 rpm, until it stalls or on the rare occasion it will settle into a perfect idle. Off idle from 850 RPM's and up it doesn't miss a beat runs smooth and perfect. (it makes me look like an idiot trying to dock....)

Here's what I've done so far, The engine is a recent repower, developed this issue halfway through the season or so last year. I confirmed the timing is dead-nuts 8 degrees BTC and it advances when I remove the AB jumper. I cannot find any vacuum leaks, checked intake gaskets, TBI, TBI adapter plate, Vac line running to the fuel pressure regulator, I did a TOTAL rebuild on the TBI unit, (injectors and all), replaced the MAP sensor. At wits end I got a beautiful used TBI from a fresh water boat and the issue still exists! All I can think of is possibly an intake gasket leak internally where I can't detect it? I haven't been able to check fuel pressure as I don't have the adapter but it just doesn't seem that a rolling idle would be my only symptom if that were the issue. I just can't think of anything else and I think I'll pull the intake and replace the gaskets just to confirm. Any ideas would be appreciated!

 

[dubs283]

possibly an intake gasket leak internally where I can't detect it?

Possible but given the intermittent nature of the issue it's not likely

I haven't been able to check fuel pressure

This spec is important but again, given the intermittent nature and based on the info provided the issue can sometimes be coaxed into occurring I'd look elsewhere first.

Have you inspected/tested the IAC and associated wiring?

 

[Quahog]

Possible but given the intermittent nature of the issue it's not likely

This spec is important but again, given the intermittent nature and based on the info provided the issue can sometimes be coaxed into occurring I'd look elsewhere first.

Have you inspected/tested the IAC and associated wiring?

IAC has been replaced with no change. It is cycling in and out while the idle is rolling, If I unplug it while it is high it'll stay at that RPM. Symptoms are the same with linkage connected or disconnected. TPS has 5 volt reference, ground and sits at .7 vdc at closed throttle

 

[dubs283]

Would look at wiring harness/connections considering component replacement and visual conformation of activity

Merc CDS/DDT is the original diagnostic equipment but unfortunately no longer supported by manufacturers. Diacom still utilizes testing procedures. If available this is the best equipment to aid in diagnosis

Based on provided info I'm guessing your setup utilizes thunderbolt ignition. The mefi controller (ecm) uses the ignition sensor in the distributor for rpm signal much like later style controllers use a cps

Have you inspected/replaced any/all tune up items? If so, hopefully mostly/all oem?

 

[Quahog]

Well I redid the intake to no avail but at least it is ruled out. It is definitely something telling the the IAC bad information. If I shut the key off and let the IAC "park" and unplug the IAC it will idle on restart and not roll up and down when revved up and returned to idle. After the initial a smooth cold start idle as soon as it is taken off idle and returned back to idle it will stall or roll. The coolant temp sensor is also new. About the only thing that hasn't been replaced is the air charge temp and the ECM.

 

[Fun Times]

Just wondering, did this seem to show up before or after doing anything with the TBI and its mounting gasket?

What is the part number of the TBI mounting gasket you are using?
The reason I’ll ask is the small possibility that while the Merc part numbers have changed, maybe there’s a slight change in how the gasket is made causing issue such as how the gasket fits, is cut to fit, holes covered/uncovered and/or the thickness one way or another has affected the operation as that has been found to be the case on some carbureted models in the past…Might be worth trying to get an old stock no longer available original part number vs the newer one and see if it helps any... Yes it's a long shot...But maybe you'll get lucky.

Here’s the new number, 27-8M0077609

New Mercury Mercruiser Quicksilver Oem Part # 27-8M0077609 Gasket 745061283916 | eBay

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Here’s the old number, 27-806640

Quicksilver Gasket PART # 27-806640 (Set of 2) for sale online | eBay

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From item number 11,

Throttle Linkage

Throttle Linkage

www.mercruiserparts.com

I honestly don’t recall the fixes at the moment to your symptoms or even if there was any posted but I know you are not the first to go through this issue on the TBI’s of reading forums over the years…If you haven’t, try doing some research online mercruiser tbi surges and stalls at idle and see if additional past info comes to light... Here's a start to that research as this member had 3 threads trying to work it out but didn't finish the results, https://forums.iboats.com/search/2659185/

The following guys said in the comments of a YouTube video:
It would be nice to know if it was the fuel pressure regulator or the fuel pump?
Both and a bad ground on back of engine block.

If possible on your end, you may want to perform an engine vacuum test too…, https://www.hemmings.com/stories/article/testing-engine-vacuum

Also what is your Merc serial number?

 

[Quahog]

Thanks for the replys, I've been a professional auto tech for 30 years and this thing has reduced me to a parts hanger.... I ordered a fuel pump and regulator seeing that it's the last two items I haven't replaced. I did learn how to check and clear codes. It had a knock sensor code, and two I think TPS codes. All of which I probably caused while assaulting the poor thing. After clearing I ran it for 30 minutes and had no codes reset just 12 on repeat. I'll report back after installing the pump and regulator.

 

[Quahog]

The video "Fun Times" posted for me had just about the same symptoms as mine and they stated that a fuel pump and regulator fixed it. I was skeptical because I just couldn't wrap my head around how that could cause my symptoms but I didn't have the tool to test it properly and they were about the only things I hadn't replaced already. So I went ahead and did just that!
Well I got the "Cool Fuel" unit all cleaned up, replaced the pump and the regulator, I even went as far as obtaining thermal grease to reapply on the pump and cooler. Put it all back together, It's in a terrible spot under the left side of the motor straddling the motor mount but actually wasn't that bad to get out seeing that I thought I'd have to pull the motor to work on it. It actually fixed the damn thing, after replacing almost everything it actually returns to idle properly. I still haven't sea tested but I ran it up down and sideways on the muffs. Thank you for your time and the help!