Engine hunting

[Don]

Hi I am hoping the collective can shine some light on my Engine problem, I have a Mustang 4.6 Modular Engine. I passed the I.V.A. in 2021 and I put it on the road. Three years later it's time for the M.O.T. Popped down to my local garage only to be told it's failed on Emissions. Check everything only to find that the Lambda Sensor was dead. New Lambda ordered and I got in touch with Emerald who made my ECU, they connected through my Computer while the Engine was running and confirmed the new Lambda was operation ok. Back to the local garage and yet another fail, two things the Mechanic told me, too much air was getting into the system, hence the readings were off, also with the extra air it caused the engine had starting hunting, this was ticking over between 500 and 900 revs. I bought a Smoke Leak Detector and found a few leaks at the silencer after the Lambda which I have now sorted but it's still hunting. I have done the Smoke Test on the Plenum and surrounding area and not found any leaks, even sprayed water with washing up liquid over the joins all round the Plenum but can't find the problem. Anybody had similar happen to them that could shine some light on this, all I want to do is drive the car.

Thanks.

Don

 

[Randy V]

I’m not that familiar with your engine, but I know I would look at the IAC and TPS.. Also the MAP sensor..

 

[Paul Hendrickx]

Hi I am hoping the collective can shine some light on my Engine problem, I have a Mustang 4.6 Modular Engine. I passed the I.V.A. in 2021 and I put it on the road. Three years later it's time for the M.O.T. Popped down to my local garage only to be told it's failed on Emissions. Check everything only to find that the Lambda Sensor was dead. New Lambda ordered and I got in touch with Emerald who made my ECU, they connected through my Computer while the Engine was running and confirmed the new Lambda was operation ok. Back to the local garage and yet another fail, two things the Mechanic told me, too much air was getting into the system, hence the readings were off, also with the extra air it caused the engine had starting hunting, this was ticking over between 500 and 900 revs. I bought a Smoke Leak Detector and found a few leaks at the silencer after the Lambda which I have now sorted but it's still hunting. I have done the Smoke Test on the Plenum and surrounding area and not found any leaks, even sprayed water with washing up liquid over the joins all round the Plenum but can't find the problem. Anybody had similar happen to them that could shine some light on this, all I want to do is drive the car.

Thanks.

Don

Question, do you have vacuum assited brakes? charcoal canister?
Paul

 

[Ian Anderson]

Is this on a GT40?
If so every slip joint can leak ehhaust gasses out and fresh air in as the evacuation works on a tuned exhaust

Sealing the slip joints suggested substance is grease which allows them to
Slip on and when heat cycled will seal and still come apart when you need to dis assemble

Or reset the idle to 1200 to 1500 when internal pressure stops it drawing in air.

Ian

 

[Don]

Thanks guys, yes its a GT40, and no I don't have vacuum assisted brakes. I sealed the joins with exhaust sealing paste when it was built which then passed the dreaded IVA. Just at a loss to find where the problem is.

 

[David Garton]

Don,
It is common to have an electronic controlled idle loose idle strategy when battery voltage is lost. There are some things to set prior to the relearn mode such as base Idle and the procedure goes like this. With engine off open the throttle body and clean the black sludge or PCV vapor stain that builds up at the butterfly to housing area. This can be done with carb cleaner and an old toothbrush. ( Not a wire brush) then start the engine and unplug the IAC solenoid and see if the engine stalls? It should not stall but probably will. If it stalls there is a small set screw that the throttle linkage stop rests against and don't always have a head to adjust with a screw driver so you take a set of needle nose vise grips and turn the screw up until the base idle is about 450 to 500 RPM with the IAC still unplugged. Once that is set you plug in the IAC solenoid and will notice the idle jumps up when plugging it in. So you have you base set at this point and will need to disconnect the battery for 30 minutes so the ECM looses the idle strategy and you are ready to connect the battery and start the car. Upon start up you will not want to have any loads on the engine such as AC. You will want to turn the steering from stop to stop a few times allowing the ECM to notice the load and then turn on the AC if equipped and do the same turning the steering wheel stop to stop. After this you should notice a stead even idle speed. That is the relearn procedure. Hope this helps as I'm sure your base is to low and when the IAC try's to drop the idle it almost stalls and the IAC ramps it up causing the hunting.

 

[Alexk]

Too much air is almost always related to a vacuum leak. I'd put money on your old sensor being good still as well. It's highly likely the old sensor was just reading the value saying that it's too lean, a lot of people see that code and change the sensor because they think it's bad.
If you can't find a leak externally then it's likely internal, somewhere in the valve cover maybe. The pcv system is drawing the leak into the engine if it's internal. Depending on what the code said, either bank 1 or bank 2, it should narrow it down.

Also, if you have any exhaust leaks near or before the sensor, it will cause the same issue. They act as a venturi and pull outside air into it which throws a false reading and makes your engine run rich.

 

[Ian Anderson]

Thanks guys, yes its a GT40, and no I don't have vacuum assisted brakes. I sealed the joins with exhaust sealing paste when it was built which then passed the dreaded IVA. Just at a loss to find where the problem is.

I would take the slip joints apart and repeal them. I would bet that is where the air is getting in and the O2 sensor will then say lean and inject more fuel which can n turn turns the whole system rich

To get a better seal they do make a double slip joint ….. picture a double skin pipe and the other part goes between the two parts

 

[Don]

Thanks again for your replies, after reading your comment about Vacuum leaks I spent some time going over the top of the engine, two things that I then did appear to have solved the problem. I had removed the pipe that served the EGR valve and blanked of the access to the Plenum, on checking the blanking plate, I re-did the gasket to ensure it was air tight, the other thing I found that I was able to get some movement on the bolts that hold the Plenum. The engine is now running with no hunting, I will now return to the MOT station and see if the emissions now pass the test.
Thanks again for your input.
Don.

 

[RayD]

Nice work Don, and thanks for the follow-up post to explain your solution.
As the owner of a Ford vehicle with the 4.6 mod motor, have been keeping my
eye on this topic, “ in case”…
Your info is for sure the stuff learned by real, hand’s-on mechanics.
…Best of Luck at the MOT!