Vacuum leak near the intake manifold

[bobo83]

I recently got my second MR2 MK3 and even though it cost me 3x as much as my first MR2, it drives worse. Some of the problems are: longish cranking before starting, intermittent unstable idle, whooshing sound when starting the car in the morning, P0420, consistently positive LTFT.

Cleaning the MAF and changing the spark-plugs did not improve the situation, so I decided to do a vacuum leak test using a cigar. I put a glove around the intake, opened the throttle and pumped some smoke into the system by first unscrewing the oil cap. After screwing back the oil cap and pressurizing the system (with my mouth), I got smoke coming out near the fuel rail / intake manifold:

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Here is a short video of the smoke coming out: https://www.youtube.com/watch?v=UoP7vY0vPOM

I would have some questions for the more experienced members:
-> Am I right in thinking that there should be absolutely no smoke coming out of that area ?
-> What is the most likely spot for this leak ? I am thinking it is the intake manifold (gasket), but could it be the fuel rail / injectors ?
-> Are there any further test I could do before I start replacing parts ?

Thanks.

[Carolyn]

It won't be the fuel rail or injectors.  There are two vacuum hoses that go into the inlet manifold, one from the PCV valve on the cam cover and the other from the little blue solenoid that is clipped to the large inlet tube.  Either (or both) could be degraded and split at the end.

You are right to also suspect the inlet manifold gasket.  They do get old and hard.

You'll have to remove the injector rail and injectors to get a good view of what's up. 

Disconnecting the inlet manifold is not too bad a job, which can mostly be accomplished from the top, except for one short bracket that is above the starter motor area towards the nearside of the car.  It has two M8 (12mmm AF) bolts one needs to be removed and the other just slackened off.

I had to do it on my car.  As I recall, I think I removed the cam cover to make access easier.  It's a good opportunity to give the cam cover a new gasket.

[bobo83]

Thanks Carolyn, I will do some more smoke tests to be sure of the location of the leak before doing any work!

[bobo83]

You'll have to remove the injector rail and injectors to get a good view of what's up. 

A question: I have never worked on a car's intake but is seems to me that the injectors in our cars spray the fuel in the intake manifold. If I remove the injectors, won't the smoke just come out of the injector ports as the two are naturally connected ? Thanks

[Carolyn]

They spray into the inlet ports of the cylinder head but, you're correct in that smoke will come out of the openings.  You can stuff a bit of rolled up blue roll or other tissue into each port.

I would think that by the time you've got the injectors and fuel rail out of the way, you might just spot the culprit.

[tets]

while I was fannying with the intake pipe, I managed to inadvertently pull the pipe from PCV to inlet loosening it which caused it to tickover like a tractor.
They aren't that tight so it could have easily come loose.

[Alex Knight]

You also need to investigate and rectify the P0420 code. This is a cause, not a symptom.

You cannot be surprised by having fuelling issues with a MIL.

[bobo83]

I would think that by the time you've got the injectors and fuel rail out of the way, you might just spot the culprit.

Spent some more time today on the problem and I am fairly confident the leak is coming from the inlet manifold. This video shows clearly that the smoke is exiting / entering (I was blowing / sucking) from the inlet manifold plastic between the two middle cylinders. I carefully looked at the other pipes and did not see any smoke.

The leak is quite severe as I can hear the air coming out. Surprisingly, the leak is still there with the accelerator pedal fully depressed (i.e. throttle valve closed). Is this normal ?

Before I start ordering parts, given that I will be removing the intake manifold, the injectors and potentially the throttle body, what parts should I refresh ? Is it absolutely necessary to replace the injectors' seals ? The throttle body seems to have two gaskets, both need renewing ?

Also, the injectors are quite oily, I presume the cam cover gasket needs renewing.

@Carolyn, did I understand you correctly that I can change the intake manifold gasket in situ ? Do I need to remove the throttle body at all ?

[bobo83]

You also need to investigate and rectify the P0420 code. This is a cause, not a symptom.

You cannot be surprised by having fuelling issues with a MIL.

The exhaust system (it's one piece including the cat) on this car has been taken from another MR2 where it worked well and passed MOTs.

I had fueling issues from day one with this new MR2: hesitation when starting the car, not so responsive throttle. Shortly after getting the car I fitted the known good exhaust and after 1K miles I had the P0420 code. That's what made me look into vacuum leaks and STFT/LTFT data.

I do hope the P0420 code is related to this vacuum leak and that the cat is not dead.

[Carolyn]

You have a used cat.  All original cats are now really old and past their best.  Cats2U do a good aftermarket one.

Air should not be coming out of the leak it should be going in as the manifold operates below atmospheric pressure.

The harder the engine works, the great the relative vacuum.

You can loosen the inlet manifold sufficiently to change the gasket without removing the throttle body.

I did already suggest replacing the cam cover gasket.  Buy OEM quality ones. (Inlet and cam cover).

A big air leak will affect the fueling, as it will throw off the MAF.

[Alex Knight]

The exhaust system (it's one piece including the cat) on this car has been taken from another MR2 where it worked well and passed MOTs.

I had fueling issues from day one with this new MR2: hesitation when starting the car, not so responsive throttle. Shortly after getting the car I fitted the known good exhaust and after 1K miles I had the P0420 code. That's what made me look into vacuum leaks and STFT/LTFT data.

I do hope the P0420 code is related to this vacuum leak and that the cat is not dead.

Did you consider that the CAT could be fine, and the sensor could be goosed?

[bobo83]

The intake manifold gasket was brittle and broken in quite a few places!



Will do a write up tomorrow after I start the car (still need to clean the throttle body and install it back), but it was a fairly straight-forward procedure.

Two questions:
1. Quite a bit of fuel leaked from the fuel rail into the injector ports ... is this a problem when I start the car tomorrow ?
2. Do I need to prime the fuel system ?

Thanks.

[bobo83]

As promised, here is a short write up:

• To work on the intake manifold I first removed all the breather & vacuum pipes and then removed the throttle body. Removing the IACV coolant pipes was by far the trickiest part.
• Taking the injectors out of the fuel rail was somewhat messy, petrol went everywhere. It might have been worth disconnecting the fuel rail from the fuel hose as a first step.
• Undoing the manifold nuts is a bit fiddly but doable.
• Once you push the manifold towards the firewall, there is surprisingly plenty of room to change the gasket. As my throttle body was removed, the manifold was pretty light and maneuverable.
• I did all this without removing the cam cover. It was windy so wanted to avoid dirt getting into the engine.
• FYI: the flame arrestor (the metal gasket between TB & manifold) is available at Toyota: 13.85£ as of Oct 2024

While I was at it, I also changed the cam cover gasket, the PCV valve and all the intake gaskets. The car did start after reassembly so that was a (partial) success. I need to drive it a bit more to see if it cured any issues.

To answer my own questions above:
1. I rotated the engine by hand to rule out hydrolocking from the fuel that leaked into the combustion chambers.
2. No need to prime the fuel system.

Some photos:
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