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Discussion Starter #1
Before i start getting into stripping parts off the engine (as its getting cold and wet outside) i would just like to run my findings past a few of you and see if you come to the same conclusion.
I have an ever so slightly uneven tick over on my 4.6 gems, but it is terrible on lpg.
So i took it to an lpg fitter today who has checked the system out and confirmed that the system is ok but the petrol side isn`t running smooth enough for this engine. I have put it on the rovacom and everything from that is working correctly, so i did a vacuum test and the vacuum is low. It is wavering between 13 & 15 in.HG @ idle.
I am thinking (and hoping) its the inlet manifold gasket, what do you guys think?. The engine was rebuilt 3 years ago with top hat liners, new cam etc.
Thoughts please.
Mark
 

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JACK'S GRANDAD
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9,235 Posts
That vacuum doesn't seem extremely low to me. Not enough to cause big issues anyway.
It could be an intake gasket causing the low'ish vacuum, so you are on the right track there.
Is it steady or does it fluctuate a lot at idle?
here are a few causes of low vacuum readings:
1. Slow up and down cycles may indicate fuel to air mixture problem
2. Low readings may indicate a leak in the intake manifold. (gaskets, hoses, etc)
3. Exhaust restrictions can cause low readings (plugged cat, crushed pipes, clogged muffler, etc)
4. Carbon in engines can occasionally cause valves to stick. This will result in transient vacuum changes
5. A burned exhaust valve may cause lower readings
6. Fluctuating readings at idle may indicate worn head components (stems, guides, cam, etc)
7. Fluctuating readings while cruising may indicate worn valve springs

Martin
 

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Discussion Starter #3
Hi
Thanks for the reply, the reading fluctuates between 13 & 15 in.Hg but it is a steady up and down movement of the needle its not jumping around. It has only done 20,000 since the rebuild so although its possible i dont think its the cam or valves, but the exhaust and cats are something i didnt think of. Looking around the net last night i came accross an article about the rover v8 and ford v6, it mentioned that the inlet manifold gaskets can go internally and suck from the sump. The test it said was to remove the oil filler cap then remove the breathers from the rocker covers in turn and block them off, place the palm of your hand over the oil filler and if there is suction then the manifold gasket is blown and is sucking through the crank. So i did this test today, looking from the front of the car the left bank was ok, but when i tried the right bank it sucked my had down onto the oil filler. So i hope that this is correct and i am going to change the gasket on monday.
There is just one more thing, in the manual it said to put some sealer in the notches of the heads where the edges of the rubber seals go, does this have to be the sort of sealer that sets or the sort that doesnt?
Thanks
Mark
 

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JACK'S GRANDAD
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I normally use black permatex for parts like that, but maybe someone else has had better experience with something else.
Does the manual list a specific sealant to use?

Martin
 

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Discussion Starter #5
The book said locktite superflex but i cant get that so was wondering if it was a setting gasket sealer or one that didnt set as i can get either, just not the locktite one
Thanks
Mark
 

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JACK'S GRANDAD
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Can you get the black Permatex over there mate?

Martin
 

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Discussion Starter #8
I done the inlet manifold gasket today and found that the rear rubber seal was split, so all the gaskets were changed and all hoses / pipes were checked and replaced if necessary. Now its back together and i have only a slight improvement, the previous vacuum readings were fluctuating between 13in.hg and 14in.hg, now it reads a steady 15in.hg (my gauge says a healthy engine should be 18 to 22in.hg and that the reading i have is due to late ignition timing) The ignition timing is controlled by the ecu and so i cant adjust this. I did the test i read on the net where i removed the breather from the rocker cover and blocked it off then put the palm of my hand over the oil filler, this is still sucking apparantly pointing to a blown inlet manifold gasket but i know now that this is impossible as all gaskets have now been replaced.
Next step i think is a compression test.
Just a thought though i have a fast road cam fitted would this make any diffrence to the vacuum readings?
 

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JACK'S GRANDAD
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YES! A cammed motor will not read the same as a rig with a stock cam in it.
That is why you can have issues with brake boosters etc with a cammed motor.

Martin
 
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