The perils of automotive diagnostics and repair

Jan. 1, 2018
Some people will almost always believe a half-baked diagnosis, and some mistakes are very easily made.

Years ago we troubleshot a Grand Prix that had run just fine until the owner’s cousin had changed the intake manifold gasket and afterwards it was skipping dead on cylinder 2, so she asked if we could have a look at it. This was an engine skip – how hard could it be? First, we checked the obvious stuff (spark plug, compression, etc.) and came up short. But what we did find was that the number 2 injector didn’t sound right with the stethoscope, so we replaced that injector with a known good one, but to no avail. We then checked the entire injector circuit for shorts of any kind and excessive resistance, pin fit at the ECM and the injector, current flow through that circuit with the injector artificially energized (0.8 amps) and ran a temporary circuit overlay. Nothing changed.

When I finally scoped the injector pulse and compared it to the others, the pulse was strangely narrow, so I called a local salvage yard and obtained a replacement ECM. No cigar. Not even close. I replaced that ECM with a second one, because the salvage yard had a bunch of them on hand and they were only $20 each. I double checked everything. This made no sense at all. Finally, I Scotch-locked that injector’s trigger wire to the adjacent injector’s trigger and the car ran great from then on with no more problems. Remember, this was an OBDI system.

This is a comparison of the actual scope trace of the narrow pulse (left) and the normal pulse (right). These patterns were captured with the old Snap-On DDC

I didn’t like that temporary fix, but one of the GM engineers who was as stumped as I was told me those early GM ECM injector drivers can each handle 4 amps, and it’d be just fine carrying two 0.8 amp nozzles. Even if it had burned out a driver and I needed to keep digging, I still had two other good ECMs on hand. One way or another, that Grand Prix holds the distinction of being a grueling fueling enigma that still has me wondering to this day.

Burning in bad info

In the world of politics, news media and other sensitive areas, some have discovered that you can repeat some supposed fact enough that most of the hearers begin to believe it, regardless of its veracity. Our customers – some of them anyway – can also convince themselves that they know what’s wrong when they have little or no useful data except the symptom. Then there are those who have a vehicle concern and somebody they know who seems to have a bit of automotive knowledge makes a superficial jackrabbit diagnosis, hopping quickly across the high points without doing much else. And don’t you love those customers who bring you some parts they want installed based on an offhand diagnosis made by somebody who either doesn’t know how to do the work or “doesn’t have time?”

Even when we begin to gather data scientifically, we can still misfire on our diagnosis, and anybody who claims they haven’t been there isn’t being truthful. For just one example among many, I would have sworn in a court of law that the left rear axle bearing was ruined on my aunt’s ‘92 Crown Vic – after all, that’s where the noise seemed to be coming from, and it changed for the worse with a swerve to the right – as it turned out, she had a noisy left front tire and for some reason the noise was telegraphing to the left rear.

Back in the early ’80s a guy wanted me to replace his carburetor because two different shops using offhand diagnostics told him they didn’t do carburetor work, but that it needed replacing. One of them even claimed to have used an ignition scope and was a tune-up shop. It was a small carburetor on an inline six, so first, I bought a $6 Delco carb kit before I did anything else. Afterwards, I did a mild throttle snap and found it dropping a cylinder under load. I identified the cylinder, replaced a bad brand-new spark plug, and fixed that one.

And then there are quite a lot of people who will play the blown head gasket card without having seen anything other than an overheating issue. I don’t know how many times I’ve heard that, and I experienced it once myself on a 1993 Camry I checked for a friend beside the road. That one had split its radiator, overheated and was puking hot, sweet-smelling geysers out of the filler neck when we refilled it and fired it up. After it came to the shop on the hook, I wanted to show the class how that kind of head gasket failure looks and smells, but all those symptoms were gone – all it needed was a radiator. Go figure.

I have a 1989 Ford Bronco that was donated because the owner believed the head gasket was blown, but it was running crappy and puking coolant out the neck because it was a 5.8L, and he had wired it up using an old 5.0L firing order. When we wired it up with the right firing order, all the filler neck geysers went away.

A few years ago, we checked a 2.4L Dodge Stratus with a horrible oil leak that a shop had pegged as a rear main seal (how many offhand rear main seal diagnoses have we seen?) and used dye to find it coming from under the corner of the head.

Those of us who teach for a living know from experience that people who already believe they have all the facts are kind of difficult to convince otherwise.

2011 Chevy HHR 2.2L Ecotec

Bearing bad news

The owner of a 2011 Chevy HHR, 22L Ecotec with 123,598 miles had spent some time and money doing his own offhand diagnosis trying to get it started – he had checked the fuel pressure with a rented gauge, replaced the fuel pump and fiddled around some with a scan tool before realizing that he was in over his head. The HHR been sitting for a few months when it came in on a trailer. They had determined that it had to be something simple and were hoping we could get it going for just a few bucks. Somebody had postulated that it might have a bad crank sensor, and they brought it to us with the hope that we’d find out it was something simple. Well, it wasn’t. This one spun with very uneven compression, and when we removed the valve cover, we found some broken roller rockers, which typically means valves had contacted pistons, usually the result of timing component failure. But the timing chain was nice and tight, and looking down into the chain area I didn’t see any looseness or shattered sliders. Could it have been over-revved enough to float a valve? We didn’t do exploratory surgery, but we sold them on the idea of replacing the bad engine with a good used one.

When we pulled the valve cover on the HHR, we found several broken roller rockers. Something catastrophic happened here, so we decided to stuff a used engine in it.

The salvage yard sent an engine for that one with a few minor differences – the fuel rail had a different shape, along with a couple milder changes. When we were done, that one ran like a top and when we fixed an A/C leak and juiced up the icebox, it even had cold air.

The Expedition and the Crown Vic

A very regular customer brought her 2001 Expedition to us with a nasty coolant leak – this one’s a Triton and they tend to protect themselves from engine damage, but she just kept driving it. The water pipe that travels through the valley under the intake had rusted through and was dumping water almost as fast as you could pour it in. Initially we just removed the intake manifold, cut the rusted-through portion of that pipe out and replaced it with a hose and some clamps along with a new intake, but when we filled it with coolant, put a new thermostat in it and started warming it up, the warming never stopped – pressure was building very rapidly throughout the system and it was evident that this one had indeed blown a gasket.        

She’s a hands-on shopper, so she did her own search for a replacement engine at a price she liked, found one somewhere in Florida, and had it delivered to the shop. It had the heat pucks in some of the expansion plugs and so I knew it came from a reasonably savvy salvage yard. I crossed my fingers, hoping they didn’t turn the engine backwards while removing the torque converter bolts! Sometimes that flips one out of time.

Since I had people doing transmissions this time around, we went ahead and jerked the transmission out first, then I had another guy remove the original engine, and we carried it on the hoist over to the area where we do component swaps.  One of the first things we noticed was the narrow pulleys on the replacement engine – apparently this one had come from a Crown Victoria or a Town Car, but I couldn’t be sure. Oddly enough, a power steering pump came with the replacement engine, and so we took that narrow pulley and put it on the Expedition’s PS pump. We also replaced the idler and the belt tensioner, because those wide ones wouldn’t work on the replacement engine’s timing cover – and we weren’t about to swap out the timing cover if we could get out of it.

Initially, the guy who put the engine in the Expedition had put the generator wire on the top post at the solenoid, and that kept the starter energized when the generator was trying to work. The starter was a casualty in this case, but it’s an easy mistake for a beginner to make. The bottom photo shows the naked grooves in the generator pulley – the A/C compressor had the same issue, but we swapped out the power steering pump pulley to have the right one.

There were some other minor differences, but at the end, that engine was sitting in the frame with a new belt and everything plugged in, and the transmission was reinstalled – having drained the transmission and replaced the filter, we needed to start it up to get all the fluid back in the gearbox. We started with five quarts and hit the key.

When we started the engine to finish refilling the trans, we noticed that it had a large vacuum leak, and we also heard strange noises and smelled something burning – never a good sign, and it wasn’t the oil smoke from exhaust manifold handprints, either. As it turned out, the guy who replaced the engine did everything right except that he made one very easy mistake. He connected the alternator charge wire at the starter relay to the wrong post, which delivered alternator output current to the starter solenoid circuit while the engine was running, and that kept the starter energized, which destroyed the starter. Thankfully, it didn’t destroy anything else. But with that heavy rubber sleeve on the wires leaving the solenoid, it was easy to make that mistake if you weren’t ultra-familiar with the wiring.

This was another easy mistake to make – put a bolt that’s just a little too long in one of these and you’ve ruined a gas tank. We used the bolts and the gasket that came with the new tank, and so this leak really surprised us – even more so when we found out the new pump was faulty.

The 2003 Crown Vic ran out of gas while the gauge was reading a half tank. We replaced the sending unit with a new one from Carquest, and the guy who did the job used one bolt that was a bit too long when installing the pump, and punctured the gas tank. We got a new replacement tank, but after it was filled with gas, the leak was worse than ever. But that leak wasn’t around the gasket, it was around the plastic grommet where the wires pass through the mounting plate – I have not seen that before. Anyway, all’s well that ends well, and there was no fire. Only a bit of wasted gas.

The Xterra

The 2001 Xterra came to us with the concern that it had quit in a parking lot and failed to start, and somebody’s offhand diagnosis was that it had jumped time. This is a dicey situation, because that one isn’t a free-spinner, but even on interference engines, a timing belt can slip enough to stop the engine without valves kissing pistons. Had that happened on this one? I asked her if she had tried to re-start it (of course she had), but she told me she had only tried once and was hoping there was no damage. We didn’t want to do a lot of engine spinning on this one in the bay for fear of possibly making a simple no-start into something worse, so we checked the timing marks first.

On this one you can pull the upper part of the timing cover, slowly turn the engine with a breaker bar (feeling for interference) until the cam gear marks line up, and then check the crank pulley for zero alignment. Well, when we did that, we found that the Xterra had NOT jumped time. We did decide to do a timing belt and a water pump while we were there, so we bought the kit, and when we got the bottom part of the timing cover off, we found that the front crank seal was leaking – no surprise on a high miler like this one.

This Xterra was right in time, but we put a new water pump, tensioner, timing belt, and front crank seal in. That seal was easy to remove but hard to re-install because the step the seal lip rides on has such a sharp leading edge – so I manufactured a seal protector to get it on there

Putting the new crank seal in was something of a demanding process – we tried a few tricks, all of which unseated the garter spring and tried to roll the lip. I kept thinking of transmission seal protectors and how I could fabricate one for this job. Finally, I fetched a soft red plastic hole plug that had been protecting one of the ports on the 2011 HHR’s replacement engine and modified the plug with my pocketknife, making a seal protector for the Xterra front crank seal that worked so well I should have patented it.

The actual cause for the customer’s concern was deep in the distributor – it’d spark and then it wouldn’t and vice versa. We didn’t want to take a chance on that kind of “maybe,” so this one also got a brand new one.

At the end of that job, we found the real reason for that no-start. The spark coming out of that distributor was a come-and-go event. We got no spark from the towers, and so, with the cap off, we checked it at the coil. On the first spin, there was no spark – on the second spin, spark was popping there, and so we reinstalled the cap and the engine fired up and ran like new.

Unwilling to trust that come-and-go spark, we replaced the distributor with a reman unit. Now she has a new timing belt, crank seal, water pump and distributor.  Maybe that Xterra will be good for a while.

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