QuestionQUESTION: Hello Howard,
When going down a hill, with no foot on the gas pedal, if the engine is holding the car back, I hear a backfire in the engine. At least that is what I call it. The engine seems to run fine when accelerating or idling. If I put the car in neutral while going down a hill, the backfire stops. I do have an oil leak from the front breather cover (the vented one) which I am trying to fix. Would that cause a backfire. I remember the engine doing this before I had it rebuilt. As is, I can't use the transmission to hold the car back on hills without having this noise. It even does it 4th gear, though not as loud. Any idea what is wrong?
ANSWER: Hi Wayne,
Backfires are usually out the tail pipe on deceleration and can be caused by several things. First is the most common and that is an exhaust leak close to the front of the exhaust system. Other causes are ignition timing off.
An oil leak at a breather pipe will not cause a backfire although an engine problem can cause a backfire inside the engine that can blow breather pipes off which then can be an oil leak. To be safe, you should have a compression test run on the engine to eliminate that one as a cause.
Let me know,
Howard
---------- FOLLOW-UP ----------
QUESTION: Compression test results were:
#1 = 110 psi
#2 = 145 psi
#3 = 155 psi
#4 = 115 psi
This following an engine rebuild, including a new cylinder head (the old one was cracked - the guy who rebuilt the engine found a used head which he tested and said was good) and pistons. Are these values acceptable? If not, what do you recommend I do?
AnswerHi Wayne,
The #2 & #3 are ok but there is too much difference between #1,4 and the other two. Be careful that you don't condemn an engine unless the compression test was done with the throttle wide open and many revolutions were run on each cylinder. In a shop we would not condemn an engine on just these readings. It just means you need to do further tests to be sure. We would run the same test a second time and if the results were close to the same we would remove the valve cover and reset the valve clearance. If any clearance was found to be tight we would run the compression test a third time. This time we would do a "Dry" and a "Wet" test. (A wet test is done by putting about 4 or 5 squirts of engine oil in the plug hole just after you do the standard dry test and do another compression test right away) You will see an increase in the pressure but it should not be more than 10% to 15% higher. For example, if the "110" cyl. goes up to "165" on a Wet test it is MOST LIKELY a ring problem. If it only went up to 115 or 120 it is MOST LIKELY a valve problem.
After the wet and dry test and we would do another test and this one is only done when all the compression tests show there is a problem. You need compressed air to do this test but it is a final and very exact test. Meaning this test results no longer is MOST LIKELY but it is 100% SURE.
You also need a fitting (available from pro tool trucks and some auto parts stores) The fitting adapts an air hose to a spark plug threads.
The method of testing is to put #1 piston as close to TDC (on the compression stroke) as you can. (timing mark)Pull all the plugs and screw in the fitting into #1 hole. Put the car in 4th gear and pull the hand brake up hard. Then apply the air hose to the fitting. Now, open the throttle and listen in the air filter intake. If you hear even the slightest hiss, the intake valve is bad. Go to the tail pipe and listen up close and if you hear even the slightest hiss, the exhaust valve is bad. Open the radiator cap and watch for bubbles or a raise in fluid level. If anything is noted, you have a leaking head gasket or a crack in the head. This test will pick up even the slightest gasket leak even before it is noted as an overheating problem.
We would open the oil filler cap and listen but here you will hear hissing. ALL engines hiss here, we learned by doing this test on known good engines and known bad ring engines to be able to tell how much hiss is acceptable. This part is not necessary as the wet test is sufficient to test rings.
Keep in mind that a ring leakage after an engine rebuild where they used the same pistons and only put in new rings can be noted if the rebuilder did not deglaze the cylinders and the rings have not seated. (They will seat within about 500 mi of normal driving) However, the 110 and 115 reading would be considered too low to blame on normal ring leakage so if the readings are correct you have something wrong. Do all of the tests and don't skip any.
Let me know,
Howard
