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Rough Idle On 4G12 Engine


GAPAUTO
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I have a Lancer wagon which has a 4G12 enigine. Two months back i have rebuild the engine by changing bearings ,cylynder rings , oil seals and valve seats and guides. Last week i droves the car which didn't have any problem. But yday when i started the engine it was rough and from the sound it seems like one cylinder was missfiring. When i rev the engine the missfiring sound goes a way.

I tried removing one plug wire at time to check which was missfiring and it seem the 1 and 2 were not firing as it should be. Change the plugins and checked the wires by replacing them with another set i had extra. But it seems those are not the problem. When i remove the wire half way on cylinder 1 and 2 i can here the spark which is jumping from wire to connection in the plug. I checked for vacume leaks and intake manufold mounting points on the head as well. Check the value clearence last week at the garage and they were ok as well.

So could this be a problem with my platinum points or the condenser ? Or is it a problem with fuel delivery or mixture ?

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Remove the plugs and see if the tips are covered with carbon and if there are oil deposits. If your rebuilding job is done in an acceptable manner you should not have any oil deposits but may have some carbon deposits. During the overhaul did you change the valve guides and plugs? How long did you use the car after the rebuild job? Some times even the new plugs may develop leaks fail prematurely.

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This means your idle mixture is too lean or the idle throttle position is wrong. My guess is the mixture is too lean. However first check if there is little pre-play on the acelarator cable and the throttle lever lands on the idle adjutment screw when you release the pedal.If not you will have to adjust the accelarator cable and the idle screw. Also there is a damper to prevent sudden closure of throttle which has to be at the right place and the throttle position adjustment for the AC unit. Sometimes the mechanics set idle speed using the wrong type of adjustment. If all these things are OK and if you can locate the idle mixture screw you can carefully turn it anticlockwise a half turn and see if the situation improves. Please note that this screw is very delicate. Just use your bare fingers to turn it if possible or use a short piece of a hacksaw blade as a screwdriver (Idea is to turn the screw with a minimum leverage).

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I suspect a sticking valve since the engine was rebuilt recently. If the mixture was too lean I wouldn't have expected for 2 cylinders to behave differently from the others (as in disconnecting any plug wire should cause the same issue).

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This car didn't came with a factory fitted AC unit, So there are no AC dampers in Carb. I checked the idle screw and the mixture screw. Both seems to be fine. Even if i raise the idle speed by the idle screw (the one which adjust the throttle possition) i can feel a light miss in the engine. I'm afraid whether its a sticking value as metioned in the above reply :(. How can i checked that without taking the head off ?

And what about the condenser and points ?

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This car didn't came with a factory fitted AC unit, So there are no AC dampers in Carb. I checked the idle screw and the mixture screw. Both seems to be fine. Even if i raise the idle speed by the idle screw (the one which adjust the throttle possition) i can feel a light miss in the engine. I'm afraid whether its a sticking value as metioned in the above reply :(. How can i checked that without taking the head off ?

And what about the condenser and points ?

A competent mechanic should be able to diagnose this rather quickly. Basically if the sparks are going through all the of the lines and the plugs are ok, you need to focus the attention on the fuel delivery system and in pre fuel injection engines which share a common fuel delivery mechanism it is mainly controlled by mechanical timing and valves.

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I can understand you predicament now. As Don says a competent mechanic hould be able to diagnose it quickly. But when you come across engine problems soon after having your engine rebuilt it is understandable that you lose confidence in the competence of mechanics. Get a mechanic who has a simple compression tester to measure the reading in each cylinder. You should get a lower reading for the ones you suspect. You did not say for how long you used the vehicle after rebuilding. If you have done close to 1000km get the tappet clearance checked and adjusted if necessary.

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Thanks every one valueable comments, I went to the same mach today and he checked the values (even though i have made around 600km after rebuild) for clearence. He adjust once value which needed to adjusted and when we fireup the engine the miss was still there. Then he checked for vacume leaks in intake manifold and check the EGR value as well. It has been blocked by the previouse owner and it should not be a problem since its blocked. Then he checked the points and adjust it a bit by making the gap bit larger. It seems that in one coner the gap was not enough. Now the idle was much better after that. But still i feel there is a slight different how my engine idles :(. i'm wonder wehther i should chnage the points and the condenser :speechless-smiley-019:

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Thanks every one valueable comments, I went to the same mach today and he checked the values (even though i have made around 600km after rebuild) for clearence. He adjust once value which needed to adjusted and when we fireup the engine the miss was still there. Then he checked for vacume leaks in intake manifold and check the EGR value as well. It has been blocked by the previouse owner and it should not be a problem since its blocked. Then he checked the points and adjust it a bit by making the gap bit larger. It seems that in one coner the gap was not enough. Now the idle was much better after that. But still i feel there is a slight different how my engine idles :(. i'm wonder wehther i should chnage the points and the condenser :speechless-smiley-019:

Changing the points and condenser will improve the ignition and give you a stable idle at least for sometime but unlikely to solve the root cause in the long run. If you have gone to the extent of rebuilding the engine now you almost have a new engine which is expected to perform to its design capacity. I do not know how you got the idea that blocking the EGR by the previous owner "should not be a problem". These are the MAKA BAAS jobs they do here. If they cannot understand how a certain mechanism work their solution is to either block or bypass it. Japs may have done years of research to design a suitable EGR for this car and our guys simply decide to block it and expect it not to cause a problem. I do not understand the logic behind it. However, EGR may not be related to your issue. But blindly blocking a EGR is most likely to reduce the performance of an engine originally designed work with it. I have seen some mechanics even removing the radiator thermostat (One on the water passage) saying " mahaththaya, meka kandata hondane!".

Why I say that improving the ignition will not solve the root cause is because in what we call a fixed draft carburetor the only two thing you can adjust - ie the idle speed and the idle mixture and hence they can be set for their optimum values. Even if you have a somewhat imperfect ignition still at idle you can deliver the best mixture to cylinders for combustion. But if you have a leaky valve or a cylinder with low compression then you would not get a smooth idle. The best way to know if you have compression issue is to measure it. Having said all this I have very preliminary question. Do you have this issue only when the engine is cold?

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I think I agree with Rumesh88. You have to treat the cause not the symptoms. To rule any issues with the combustion chamber out (leaky valves, sticking valves, leaky chambers etc) you probably should do a compression test and probably get your valve clearances and tappets adjusted. This only requires you to take off the tappet cover.

Did the 4G12 have an EGR valve...? I don't think I've noticed an EGR on my 4G91 though I might be mistaken.

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This happens even when the engine is hot as well.

Well some thing i notice is that as soon as the car is started it runs really fine for few seconds (~ 5secs). Then only the idle get rough.

Well its has a EGR value which has two diaphrams, But all hoses are disconnected. I agree with you on some mechs disconnect things because they are lazy to fix :(.

I will check the distributor cap as well, i clean it well , but i didn't check the connection points when they have a good connection. Thats i will check as well.

The nice thing is before the engine rebuilt, two of my exhoust values were not even seated well. But the engine didn't have a problem like this, even though it was bit rough on idle and fuel consumption was high but not like this :(.

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You seem to have a issue with compression. Please get it measured. It is a simple procedure. All one needs to know is how to connect, read it and to keep in mind to turn the engine for a similar number of turns before taking the reading for each cylinder. If the rebuilt job is satisfactorily done, the compression difference between cylinders shall not exceed 5%. (In fact, to measure and compare compression to this accuracy you need to do it simultaneously for all cylinders, but even with individual cylinders measured one at a time should give you a reasonable reading). It is only then you can achieve the required balance between the cylinders for a smooth RPM at low revs.

Did you get your engine head faced? If not there is a likely hood for a gasket leak even though you have a new gasket now. Do not rule out this possibility too. You may not have experienced the same before you rebuilt even with leaky valves because of an obvious reason!!! What you had then was four bad cylinders but since all four were bad they were in balance. Please keep in mind if you have three very good cylinders with high compression and one bad cylinder with low compression, then it is more difficult to achieve the cylinder balance than with an engine with all cylinders with low compression. This is because in a four stroke engine, the bad cylinder would have to do more work during its power cycle to pressurize a good cylinder which is in the compression cycle. So the effect of cylinder balance is synergistic. I hope I made my point clear.

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