Tech To Tech

Tech to Tech
by Mark Warren
Working With Check Sheets
Forming a game plan is central to a successful diagnostic procedure. With a plan you know where you are going, where you have been, and can then evaluate your procedures. So how do we start? Let's consider a recent repair in my shop.
Check sheets are essential for tracking a repair and saving time. Consider the tech who has spent two hours working on a car and now seeks help from the boss or co-workers. Does this sound familiar? How is the fuel pressure? OK. The fuel volume? OK. Did you check this while the problem was happening? No. How about peak KV? Looks OK. Burn time? OK. EGR system? OK. Exhaust back pressure? OK. Burned through rotor? Don't know. How about grounds? How about when another co-worker comes in and the questions start all over? What about when the service writer has to write the bill and the 20-question drill begins again. Zowie, you may as well just burn money!
A good check sheet can eliminate many of these questions and more can be answered in seconds. My techs were always relying on me for more options when they hit a diagnostic wall. With a good check sheet most options are spelled out on one page.
The first check sheet is the customer interview sheet. The customer's memory needs to be jogged to get a comprehensive list. Let's go to the example. The car in question is a 1992 Suzuki Sidekick with a 1.6L 16-valve engine with a MAF sensor-style, fuel-injection unit and 66,000 on the clock.
The complaint included constant "pinging," especially under load. There was also gradual loss of power after two hours of driving. When the problem peaks, engine power is approximately one-half of normal.
Information from the customer was that the problem developed slowly over a one-year period. Also, this customer is knowledgeable about vehicle maintenance and does much himself. He brought a factory manual and has all repair orders for past work.
Several things had been done with no improvement:

Under warranty, the No. 3 injector had been replaced three times to repair a misfire. Later the customer found the source of the problem, a No. 3 injector wire shorting on the back of the head. He repaired the wire.
The entire exhaust system had been replaced. The customer has a fabrication shop and made headers and the exhaust system. A high-performance muffler was installed. The converter was factory and new.
The customer installed an aftermarket air filter replacing the original air box. No improvement.
The customer tested compression, with a reading of 200 PSI on all cylinders. Yes, this is in "spec," according to the factory manual. This engine has a factory 12.5 to 1 compression ratio.
The customer checked timing, which was OK.
The computer had been replaced under warranty. It had a mismatched code indicating it was a California computer in a federal spec car.
The customer had adjusted valves to spec at specified intervals.
The plugs and fuel filter were new. The plug wires and cap and rotor had been replaced with original equipment (OE) parts.
The intake manifold gaskets had been replaced twice on the guess of a vacuum leak.
The customer had replaced the timing belt. No change.

Our game plan started with a "pinging" check sheet we've developed, combined with information from the customer. The testing included:

Check timing and advance.

Check EGR system.

Check knock sensor circuit. (Maybe the pinging gets worse as the engine heats up and the knock sensor has to roll out more and more timing, creating the loss of power.)

Double-check compression and specs.

Check for a heated air intake defect causing air that is too hot being injected by the engine. (This is why the check sheet is important, this one is often overlooked.)

Use Motorvac equipment to clean combustion chamber deposits.

Consider if a lean condition is causing loss of power and ping.

Check fuel pressure. Many fuel pumps fail when they heat up. Remember the earlier question asking if pressure was checked when the problem was happening? Just testing the pressure in the stall is not enough!

Check coolant condition and engine temperature, also considering localized overheating problems.

Check for an exhaust back pressure problem. Highly unlikely considering the work done, but never say never.

Check for codes.

Make sure TDC marks are true TDC relative to No. 1 piston movement.

Check possible vacuum leaks.

As mentioned, this check list is a combination of past experience relative to loss of power complaints and information from the case at hand. Your list should come from as many sources as possible -- experience, manuals, co-workers, tech lines, on-line ideas or the customer. Once you have all your ideas, make two lists. The first one in order of the most likely cause first. The second list uses the same problems listed in order of the easiest to test first. Knock off the easy stuff first, such as timing and visual checks.
I started with the factory manual provided by the customer. He was correct about the compression spec. I noted the correct timing spec. I also checked the EGR system operational information. This was a positive pressure feedback system. I also checked for the location and operation of the knock sensor circuit; this eliminated one possibility. This engine has no knock sensor. Darn! I had high hopes for that one.
Information in hand, I finally went to work. The base timing was right on. The total advance at 3,000 rpm was 36 degrees, which seemed OK. The EGR did "bip" open on snap throttle. I checked the intake for vacuum leaks using a Vacutec, no problem there. I wondered if the low-flow exhaust system had affected the back pressure, causing insufficient EGR. However, the problem existed before the exhaust work. Hmm ... just another possibility. Checked no codes, no surprise there. Double-checked the underhood sticker and confirmed it is a federal car. Coolant tested OK. A check of the O2 sensor with a voltmeter showed it sweeping OK at idle and at cruise in neutral in the shop. Sure, I had not duplicated the problem, but I wanted the easy stuff out of the way.
The visual check includes checking the underhood vacuum sticker against hose routing. I once spent three days on a problem where two hoses were switched. The EGR vacuum source was on the correct location on the throttle body. Checked the EGR solenoid -- bingo! The hose connections are incorrect. There is a push-on filter on the top nipple and the hose to the valve is on the bottom. The vacuum diagram shows this is incorrect. Reversing the filter with the hose caused the EGR to open much further. Ha! Another fix from good information and a logical approach. No need for a test drive. It's an obvious fault relating to all symptoms. Just let the customer drive two hours to confirm if the power loss is gone.
The customer returned in the morning. The ping was gone, but the engine had no power at all now. He had checked the EGR diagram in the manual and it contradicted the underhood diagram. EGR was on way too much. It turns out the manual was correct and the underhood sticker was wrong. So much for good information! This shows the need for multiple sources of information. I was chasing a ghost! Oh well, back to the beginning. At least I have plenty of options to check.
OK, remember to begin at the end. I still suspected the EGR system, so I went to the end of this system. I connected a vacuum gauge into the EGR line between the modulator valve and the EGR valve. This confirms the operation of the entire EGR vacuum system and tells me if the low exhaust pressure is giving me trouble. So vacuum gauge in hand, I was ready for a test drive. Keep in mind that most EGR valves are full open at low vacuum readings (4 to 7 inches of vacuum). This is because the EGR may need to be open under high load conditions when vacuum is low.
On light acceleration, the valve had 4 inches of vacuum. Under light load at a steady cruise of 40 mph, the EGR vacuum dropped to 2 inches. This seemed kind of low. I decided to check engine source vacuum to be sure there was enough to supply the EGR valve. The engine vacuum was 2 inches at a steady 40 mph cruise! An engine vacuum problem was causing insufficient vacuum for the EGR, causing the ping!
In next month's column, the cause of the engine vacuum problem will be explained. Sorry for the cliffhanger, but going through this procedure step by step gives an idea of how check sheets are used and adjusted as dynamic diagnostic plans.
About the author: Mark Warren is the owner of Mark's Auto, an Automotive Service Association (ASA) member shop in Tucson, Arizona. Warren discusses underhood diagnostics and other service considerations monthly in Tech to Tech. If you have a comment for Mark Warren, you may write him c/o AutoInc., P.O. Box 929, Bedford, TX, 76095-0929, or fax to (817) 685-0225. Mark's email address is 76727.3631@compuserve.com
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AutoInc. Magazine ®, Vol. XLIV No. 9, September 1996