Compresion Testing Results

[megadodo]

My Tin Top Scenic has started to run rough and consume more expensive jungle juice than it should. I have sparks fuel pressure no inlet or exhaust leaks that I can find so ouit with the compression testor. Results after 6 spins of the engine

 

Cylinder 1 200 psi

2 200 psi

3 185 psi

4 175 psi

 

Is this enough of a problem to cause very noticable rough running or Iam I perhaps missing something else? Wouldn't a blown head gasket between 3 & 4 mean they would the same reading and a lot less than the other 2?

[Guest garyfelton]

Hi, what engine is it? Would like to help as I am an ex Renault mechanic.

 

Regards Gary

[megadodo]

It's the 8V 2 litre, converted to lpg 50k ago so I suspect valve recession but i thought (hoped really) the compression measurements just weren't "bad enough" to result in such rough running? Engine now has 118k on the clock.

[Guest garyfelton]

I think valve recession is the probable cause as I have seen this before, first thing to check is valve clearance, if no clearance then this will confirm your thoughts. I have recently had this problem on a Clio with only 56k on the clock, compression only down slightly but after resetting valvle clearance ran ok. On your engine adjusting the valve clearance is a problem as it is done using either shims on top of shim buckets or the buckets themselves. HTH Gary

[megadodo]

Thanks for that Gary, I'll check the clearances tommorow, I bet there is a Renault tool for changing the shims?

[MarkBzero]

I think Gary is most probably right with that mileage. However it could be a number of things have you or somebody got access to a compressor it might be worthwhile doing a pressure test with a old spark plug and a welded air union. (you can use a electronic tyre compressor with the same results, but I have never made the union to fit) Remove the valve cover make sure the valves are in the closed position i.e compression stroke. Use the air fitting in the spark plug hole and add compressed air (carefully and slowly. Now listen for a loud hissing from one of the following: 1. Crank case 2. Intake manifold 3. Exhaust pipe 4. External cylinder head/block interface 5. Header/expasion tank.

 

Wherever the hissing is coming from will tell you where you are losing the compression. Hope this helps.

Regards

Mark

[megadodo]

Cracking idea, I'll give that a whirl as well! Always up for new ideas, although the wife doesn't like them all!

[megadodo]

Having checked the valve clearances it does look like valve recession as I can't get a feeler gauge into the cam/follower gap at all on the low compression cylinders exhaust valve! Gary, do you think its worth re-shimming or will the valve seats need grinding?

[Guest garyfelton]

There are several Renault tools to aid shim changing but to be honest they are cr*p and don't work on all the 'F' series engines. as I mentioned before some even use the bucket as the shim. The tool to change just the shims pushes down on the bucket allowing you to remove the shim which sits into the top of the bucket. You may be able to use a lever on the edge of the bucket to push it down but the springs are quite stiff and getting the shim out is fiddlely with the renault pliers, as the oil tends to stop you lifting the shim due to suction. If you are going to remove the camshaft to reshim the tappets, then this will invovle removing the cambelt & cam pulley, and if you're going that far then it may be best to remove the head and do the job properly if you intend to keep the car. All a matter of cost really. Shimming the tappets can be a pain as if you have no clearance in the first instance, then it is guess work on how much smaller shim you try first. If the valve seat has worn too bad you may not find a shim small enough to give you the correct clearance. I have had to have shims machined in the past to get the correct clearance.

 

HTH Gary

[Guest old_timbo]

As Mark says, do a leak down test before you start taking things apart. This will tell you whether your valves etc. are sealing OK. I had a go at this today on my Pinto after getting some very odd / scattered compression test results. I found no leakage at all through the valves and only 5 - 10% pressure drop past the pistons (up to 20% OK). So the next thing to look at will be the valve clearances to see if I can get consistent compression pressures. I got very lucky and found that a standard airline connector screwed in to the end of the adaptor on my Gunsons compression tester, so it was easy to do the leak down test.

 

Tim

[megadodo]

Exhaust valve on 4 definately not sealing! I've taken the car off the road and I'll re-shim the cam at my lesure. If that solves the problem, great, if not its on to ebay for another head/engine. It will ony cost me the price of some shims and my time. In the meantime I've got myself a 318is to play with!