Before going to the trouble of pulling spark plugs and running a compression test, the thought of connecting a vacuum gauge might be considered as that could provide an indication about low compression, camshaft out of time, etc.
There is no hard, fast number for engine manifold vacuum during the test as that will vary based on engine condition, barometric pressure, altitude, and so on. Generally speaking, the number on an engine with no problems is from 17-21" of vacuum at idle. The gauge reading should be rock steady with no surging or twitching of the gauge needle.
The vacuum gauge is cheap, easy to use, and unfortunately; not used often enough in my opinion.
It’s not the answer to every problem but is a very good start.
Ask a pilot about manifold vacuum on a piston engine aircraft and they will tell you how important it is.
NEWS FLASH: I have an appointment for next Wednesday morning, for the mechanic to diagnose the problem. I am perplexed by the attitude of the intake guy there, the one who asks you what’s wrong, documents it and documents what the technician finds, the one who gives estimates and so on. The record keeper, customer service guy.
I told him that the mileage had gone down immediately after the issue we have been discussing for several days now. From 25 MPG to 20.5 MPG. His initial reply was that old cars get lower mileage than new cars. I countered with the fact that the drop was over a period of a few days, rather than over the 10 years’ age of the car. Not a gradual, or a small, decline. Five miles per gallon is not insignificant.
Then, his reply was that cars nowadays have so many sensors and the computer is so all knowing that if anything, anything at all, were wrong internally with the engine, the Check Engine light would come on.
I couldn’t believe my ears! I had thought he was an astute technician; evidently he can talk a good game but perhaps does not understand some things about engines. I asked for a compression test, and he hemmed and hawed and kept on with the litany about magic sensors. He did not suggest that perhaps the SENSORS WERE BROKEN. You know?
So, he says they will take it for a drive with a computer hooked up to it, so they can get “in-line” data, and see if they notice the lurching that I have noticed, the feeling that the engine does not always want to pull, for whatever reason. It is quite pronounced sometimes, the lurching. Occasionally makes me want to park the car.
Yes, please keep us posted.
In the meantime, please bear in mind that “service writers” (which this person surely was) frequently know almost nothing about cars, and are best known for spouting babble that has little connection to reality.
This person has undoubtedly been schooled in how to scare customers into fuel system flushes and other superfluous (or even useless), overpriced procedures, but may not even know what a compression test is. The bottom line is that this person–who may have been selling shirts at Macy’s last week–should not be trusted regarding any technical info about your car. You may have to demand to speak with the service manager who–hopefully–has some actual technical knowledge.
Some of them were actually mechanics at one point in time, but most of them don’t know how to actually diagnose and repair cars, except perhaps for oil changes and small tuneups on their own personal vehicles
I’m sorry if I’ve insulted any CMAT L1 service writers reading this
It’s a small shop. The guy who owns it has his name on the sign, “_____'s Auto Clinic.” He is there most of the time. The few times I have spoken with him, he is usually noncommital, but I have taken that to mean that he knows enough to say, “I don’t know,” which often means a person knows a lot. He trusts his mechanics, or at least that is the determination I have made.
I believe that when I pick it up Wednesday, it will be best to speak with _____, to have the most accurate explanation.
The car only has 61,000 miles on it, and I have put about $2500 into it in the last 7 months (tune-up, front and rear brakes, new tires, new struts, new suspension assembly because it was bent from a wreck I had in 2005, and the timing chain debacle). I was planning to get it repainted and some body work done to it. All of this because I believe Japanese cars are supremely well made and I want this to be my next-to-last car or so (I’m 50 years old).
I also carry comprehensive insurance coverage on it, through State Farm. I did a car valuation on AutoTrader (I think it was them), and answered all their questions honestly. There were a million questions, and I answered all of them. Despite it having been wrecked and having 3 doors replaced et cetera et cetera, they gave me a value of $3400. But now I have put $2500 into it and the residual value is thus $900.
Well, the service ticket says it was my “mass air flow meter”. I thought that was a sensor, but I suppose it meters the air flow, right?
The Labor Description (1 hour at $105) states: “Engine performance diagnosis. Road test vehicle as necessary to verify condition related to fault exists. Perform scope tests and checks of ignition, fuel, and related system components as required. Estimate and advise of needed repairs.”
The service writer said the sensor was reporting 2 cm of air at idle, which was the “same as having the car shut off.” So, he said, it’s “running lean.”
Since the repair was going to be $278 and my money is slim, I asked whether operating the car for a few more months would cause a major mechanical malfunction. It’s been this way since my cam-reclocking repair…remember that? That was in September, so I have operated it 3 months like this. I calculated what the extra fuel is costing me, and I came up with $15 per month extra gas.
The service writer replied that operating it for a few months might foul an injector or might cause another sensor to fail, but it would not cause mechanical trouble within the engine.
Does that sound right, provided all the discussion we have had before? Is a reportedly badly malfunctioning MAF meter enough reason that I should go ahead and pony up the $278?
If you don’t you’ll pay it later anyway but you’ll have to add the cost of the extra gas as well as any damage it might do. Running cylinders lean can cause too hot a chamber and lead to spark plug damage and even preignition. It’s like using a bellows on a fireplace…the fire gets hotter. Try to postpone replacing it and the cost will grow.
I’m with mountainbike. Running the car persistently with the air/fuel ratio off will just cause avoidable problems.
I will add that MAFs can be cleaned, and quite easily on most cars. And if one can clean one and it doesn’t work then it’s not too far to being able to replace it. You could come in a lot lower than that.