Doing a compression test on a 308i, should I remove all plugs or do them one by one. Thanks Marc-André 80 308 GTSi
It's a little easier on the starter/battery if you remove them all (since the cylinders don't seal perfectly, there's a net loss in the compression-expansion process), and, if you are doing a "warm" compression test, you'll shorten the test time so the temperature change from start to finish will be less. (Don't forget to put the throttle plate at the WOT position.)
I've allways removed the plugs,takes the pressure off the starter.Engine will spin up faster ,for a more accurate reading.Also pull off the coil wires to the distributor.
WOT = wide open throttle. Of course you need to disable the fuel system (I wasn't trying to list all of the necessary steps -- just an easy one that sometimes gets forgotten). Easy way is to remove the fuel pump relay or fuse. Also, good idea to disable the ignition system, as the Captain mentioned, to not needlessly overtax the battery (but I'd unplug the Digiplexes on a 308i, to stop the primary coil currents from flowing, rather than just removing the coil secondary wires to the distributors).
Thats pretty much what I thought, but I felt it might be a good idea for me to embarrass myself anyway.
I don't know squat about the 308i, but I would be surprised if at crank speeds and wide open throttle, there's no "clear flood" function on the engine control unit. That has been around since the early eighties on most injected cars. Otherwise how would one pump out and clear a flooded engine? Can someone confirm the absence of a "clear flood" function in the ECU? I'm really curious now. Tom Kizer I'm now a vintage carbureted car guy, but also a retired auto industry fuel injection design and development engineer.
No, K-Jetronic (either without, or with, Lambda), as implemented on F, has no such advanced function -- if the key is in pos III (starter engaged), the fuel pump runs, and fuel is delivered to the injectors based on the deflection of the fuel distributor plunger (which is a function of the amount of air entering the engine and the control pressure) and via the cold start injector (as long as the Thermo-Time Switch remains closed). If things are working correctly, and the operator follows the operating (starting) instructions, it should never flood -- what can go wrong with that
Steve, some people squirt a little bit of oil in the cylinder before crancking the motor for better seal rather than crancking it dry, is this necessary? Thanks.
I guess I just find out why the car had an manifold overheat issue. Front bank is equal at 170 psi except for cylinder #1 at 150. Rear bank (overheating) cylinder #4 reading is a miserable 40 psi after 8 cranks. head overhaul are just a valve adjustment ? Marc-André 80 308 GTSi
Cranking dry is what happens every time you start the car. The way I was taught, you do an initial compression check. If the numbers are not optimal you can put a bit of oil in the cylinder. This gets into the rings and a second compression test will tell you if the loss is at the rings (compression will improve with oil) or the valves (compression will not improve with oil)
When I did the compression test on my 348 I: Removed all the spark plugs Disconnected the coils Disconnected the fuel injectors Held the throttle wide open while cranking the engine 5 times It is best to do the compression test after the engine has been warmed up fully.
+1 to what PV Dirk said -- it's (adding a small amout of oil and rerunning the compression test) is more of a technique to confirm bad rings (rather than SOP for doing a compression test). Marc-André -- It's possible that it's just a (lack of) valve clearance problem, but the trouble always is that, if the engine has been operated in that condition for any length of time, the valve sealing surface or valve seat sealing surface will get "burned" (eroded) -- so I wouldn't be hopeful that, if you found a lack of clearance situation, correcting it would make everything A-OK. After that poor compression test result, you'd probably be better off to do a leakdown test (which is better at determining if the problem is intake valve, exhaust valve, or rings) -- if it is a valve, then you've got to pull a camcover and check the clearances.
Yes, you only add oil if your compression is bad. And then it gives you a bit more information. If you put oil in first you could get false readings because if your rings are bad the oil will hide that problem. Test your engine without adding oil the first time, and if your compression is bad you'll want to pull the engine apart anyway or as suggested a leakdown will give you valuable information. But I would like to add that your engine is probably fine. Have fun collecting the information.
I've just done my homework. Adding up oil and I came up with the same results. 40 PSI. Now that I think i've narrow it down to the valves, I think, strongly, that the exhaust valve night be the issue, without a leakdown test. I'm having a intake maniflold glowing red issue, so it should be the exhaust valve. Is there a magical product that I could poor into the cylinder, flooding it to get to the valves and remove any dirt between the seals and the valve. Also, before starting dismantling the poor thing, is there a visual that would let me know my valve adjustment are out of sequence. Thanks for all your help. I think i'm going somewhere with your valuable information (that quote should have come from me). Marc-André
Occasionally, when I have run a compression check, a little piece of carbon breaks loose from the spark plug threads when you remove them, and it gets caught on the valve seat and is smashed there. Tha valve not fully seating results in low compression. More than once I have replaced the plugs, started the engine, gave it a quick jab on the throttle, then shut it down and repeated the test to find that the compression was normal. The combustion gases burn off and blow out the carbon. In your case with the glowing manifold this probably isn't your problem, but is worth considering.
If your only reading 40 psi, i suspect you have quite a bent or a seriously burned valve. If you had access to a fibreoptic I would have a look into the cylinder down the plug hole to see what you find. if not..... Easiest thing to do is remove cam cover ensure cam shaft is positioned for cylinder 4 exhaust closed. Also makes sure you 'have a cam/ valve clearence issue. Assuming no issue.... Remove the rear exhaust manifold and get a good mirror or small video camera (your phone) to see what the valve looks like through the exhaust port, shine a strong light down the spark plug hole and see if you pick up light coming though the exhaust post. If yes you don't really have much option. If no light you could try pouring some toluene or carb cleaner through the exhaust port and leave it sit for a while to see if it makes any difference. Or alternatively fill the cylinder through the spark plug hole with a suitably strong solvent - with your exhaust valve slightly open and see if it dissolves any crud.... you relaly need to know what you are doing if you go this route as you could destroy the engine if you dont ensure the solvent is removed before cranking the engine over (Hydraulic lock) If none of the above work then I am afraid only option is head off. On the positive side at least it is the rear bank, in the grand scheme relatively easy, engine in job! One other thing to consider, is it the entire header that is glowing or just around cylinder 4?, if it is all there is a risk that your entire bank is running lean due to a fuel/ air mix issue an dcylinder 4 was the first victim? If that is the case you could have other issues. Other questions.... do you have any exhaust popping or misfire?, do you hear any misfire through the air inlet if you remove the filter? Enough help in this forum to get you though it if you post pictures and findings as you go.
Tom, You ask the question, "Do you have a miss fire in the air inlet"? I don't mean to jump in to the thread like this however I have issues with fuel misting/ bubbliing out of the carbs on the right bank 5-8.. Seems like the carbs are flooding and spewing fuel but the car runs damn good !! I havent done a compression test or tried to varify if the valves are seating on the compression stroke hense why it may be blowing fuel back out of the carbs. The start of this issue was I think after the engine overheated but not sure? Ideas? Thanks Tom
Are you running a new little high torque starter?? I ask that because we started chasing a low compression reading last year on one of our Lotus twin cam motors after it's first event. It ran great but compression tested low. After not being able to get what we thought we should be getting with any of the tricks we normally employ, I called our motor builder (Huffaker) and he said that it common to see low compression reading with a high torque starter (Tilton). Made sense as there is only so much current available - when you increase torque, you lower rpms of the starter final drive... lower rpm makes it harder to get a good compression reading. He advised we ignore compression, do the leak-down and adjust valves. Thats what we've done the last few events and everything is tops! This winter with the motor out and the trans off for rebuild, I had to see so I put a stock starter back on and sure enough, the compression readings came back up. So speed matters when you are checking compression. Leak-down is a more accurate assessment IMHO. Rick
