Intermittent failues are such a pain


When troubleshooting a problem on anything, not just cars, intermittent failures are by far the biggest pain in the chair.

In fact, dealing with intermittents quickly separates the men from the boys and the women from the girls. Taking an intermittent problem in a car to most mechanics at $100/hr is a real good way to waste a lot of money, just as the average tech in the electronic factory where I worked had no chance to work out an intermittent.

When my 2002 Sienna, now parked, was fairly new, I got an intermittent evap failure. Sometimes it would work for many weeks, then fail steadily for a few weeks. I made notes and kept searching the Internet. I did NOT take it to a mechanic. I was aware there might be a mechanic who had learned what causes that problem, but it was my money and I was not giving up easily. I know how furious I would be if I took it in and got a bill for several hundred dollars with a NTF note on the bill. I can do that for free.

Finally, a man on Sienna Chat reported that he had the same failure on one of his two Siennas, and he swapped stuff until it followed the canister assembly when he swapped it. He said it looked like there were self-actuating components there, and they must have been getting sticky. I took it to Toyota in Pharr, signed the waiver that said I was responsible if it didn’t fix it, and it has given no more problems at all.

At that time, when I posted that tale here, one of the mechanics commented if a customer came in and told him what to replace, he’d hand him the tools and tell him to do it himself. In my experience, techs with that sort of attitude were ALWAYS very poor diagnosticians.

That of course is a mechanic’s decision to make. But, after 31 years as a diagnostician, I do not feel a need to apologize for making the diagnostic decision myself. I worked for some years on ‘black boxes’ (they were actually gray) that had a standard cost of $500,000 USD, due to very low quantity. (As I typed that dollar number, I hesitated; that is a lot of money for one box. I thought again, and yes, that is what they told me.)

This issue of intermittents came up again in recent times. We bought a new MABE refrigerator. I think Mabe makes a lot of refrigerators for other brand names in the US and Canada, but sells them as Mabe here in Mexico.

From the time it was new, it has had a strange problem. For weeks it will maintain temperature fine. Then, one day it either starts freezing foods in the main cabinet, or lets it warm up to around 50 degrees. If I readjust the controls, it will run fine again for weeks, then goes the other way. Usually, it’s just a nuisance, but I have been sick and needed yogurt for medicine. Letting yogurt get too hot kills it.

So we called a repairman, the same one who earlier replaced the compressor. But, the compressor was not intermittent and he handled it well.

I told him what we had seen and it soon became apparent he was the lower echelon of diagnosticians who MUST SEE IT FAIL HIMSELF! Pathetic. He hinted he knew it had to be in the control assembly inside the main box, but would not replace it. I was more than willing to pay for a new control assembly, just to see if we can fix it. I realize a lot of car customers who know nothing about diagnostics will be furious if you replace something with a NO-FIX, but I worked on customer repairs for over 31 years. And, I well know that in some cases on an intermittent failure, you simply have to eventually try the most probable component.

He instructed me not to touch the controls, but to call him when it took off again. A few weeks later, it ran up to 50 degrees again. My yogurt was at risk, so I cranked it down again. It would be several days before he came, and it’s an all day trip to buy more Sugar free yogurt.

I decided I was not going to do any more business with him, at least not on this problem. Just how much suffering will I experience while waiting for him to be satisfied that he has seen the failure himself?

By a strange quirk, the temperature has been almost perfect, around 38 degrees for weeks! So, I am guessing there is a mechanical binding or something in that control and we just happened to twist it just right.

If it starts failing again, we will shop for a new fridge, and ask the cousin who is a dynamiter to blow it all over the hillside… Just kidding, of course, but wouldn’t that be sweet?


“one of the mechanics commented if a customer came in and told him what to replace, he’d hand him the tools and tell him to do it himself. In my experience, techs with that sort of attitude were ALWAYS very poor diagnosticians.”

I don’t believe it was me who said it, but I vaguely remember that comment

In any case, I’d like to shake that man’s hand :+1:

He has a good head on his shoulders, and he shows good judgement :blush:

If you let the customers do all the diagnosis, then maybe you’d better close up shop and go jump off a cliff, while you’re at it


I dunno. I’ve always kind of looked at it as a partnership between me and the mechanic. If I can help pinpoint the symptoms and what has been tried and then together decide on a course of action. Sometimes it works and sometimes it doesn’t. I’ve been amazed that the guy has found the problem and disappointed both that none of us can solve the problem. I’m not trying to second guess anyone, it’s just that I have skin in the game and have done a great deal of thought before it ever gets to a mechanic.


Been on intermittant power with my 90hp motor for 4 years. Most of the time it runs fine, sure had it checked out by the boat doctor, compression a little low in one cyl, but every now and then it is 75%power, so I can low speed for a bit, hear it kick into full power and life is good. Now today took a few people tubing, no problem, full power was back, but yesterday, it was at 3/4 power for most of a lake cruise but kicked into full after a while. It was at least 20 outings no problem, but the problem was there before. Got a few spare parts to throw at the critter, but when it runs fine what the heck. Curious coincidence, motor runnung less than par, throw some sea foam in the gas, that fixed it yesterday, 4 cyl 4 carbs, been there on the previous 100 hp evnrude, preferring not to go there.


I don’t remember if I was the guy who made that comment or not but it certainly sounds like something I would say and something I agree with…


As a TV tech in the '70s, intermittent problems were the bane of my existence. The other tech would always take the device to the shop, find a few tubes to replace, and when it worked for a day, declare it fixed and returned it to the customer. I finally learned to just take the chassis out of the set in the home and patiently probe with an insulated stick until the problem occurred. Once you could make the problem happen, you could find places where it was easier to make it happen and finally find an area that was so tender that you barely had to touch it. A close inspection in that area would usually reveal a bad solder joint on a printed circuit board. Often I would point out the break to the customer and then re solder the joint and we would never hear from that customer again. (and I mean that in a good way)
The problem with taking it to the shop is that the jostling of moving the device would “fix” it and it was easy to kid yourself that the perfectly good tubes you replaced “fixed” the problem.


Be aware that on a multi carb 2 stroke if one carb is lean it will fail to lubricate the cylinder.


Unless I am so certain of my diagnosis and repair that I will guarantee the problem is solved and if I am wrong make subsequent repairs totally free, parts and labor, until the problem is corrected I can’t object to a car owner insisting on considering their opinion. If the customer’s guess is reasonable and the problem is intermittent I have often followed their suggestion and hoped for the best. Especially when doing so was cheaper than an hour of diagnosis time.


At some point a mechanic should show some professional pride and diagnose the car himself

After all, that’s his job

If he can’t or won’t do that, then maybe he should sign over his business to the customer . . . the one that found the repair on the internet

With one big clause . . . “Don’t bother to call me when you can’t figure something out”

I sometimes tell people “If you think you can do my job better than me, prove it. Give up your current job. Become an apprentice for a few years, or go to evening school. Get hired somewhere, spend tens of thousands on tools, and THEN tell me you’re doing a better job than me. If you’re not willing to prove it, shut up.”

But when I tell them that, they’ve had it coming. Patting themselves on the back, talking about how their $50 code reader told them the solution. Talking about how some guys on a very questionable car website . . . NOT cartalk, BTW . . . had the answer, how the teenage kid next door knows everything, and he said such and such, etc.

I would never dream of telling a trained craftsman that’s working on my house or whatever, that I know a better way to do his job.


DB and OK, please tell us if I brought my Sienna to you and told you the evap was very intermittent, exactly what would you do and how much would the bill be? I’d like to know.

I think most places it would cost $100 just to read the code that I already read a zillion times. $100 down the tube and nothing gained, right? Not even picked up a wrench. $100 already.

Now, you two guys are unusually good mechanics, so maybe you would know that intermittent evap on the Toyota Sienna is often the canister assembly. Would you suggest that to me, and explain it is an intermittent and the only way to be sure is to try it? Probably not, as someone said gotta’ be sure, so a couple hours more of labor, getting close to what? $400 and nothing gained? A no trouble found and pay my bill, $400 wasted. Just how many times do I go through this sequence, all based on your ego problems and my car still not fixed when I already know what the problem is?

But, if I took it to your shop, would you be the mechanics to work on it? Or, would the boss say, “Put the new kid on it. He needs the experience?” And, second trip, the boss would say, “Put the kid on it. He worked on it before.” People of modest income simply can’t afford your viewpoint.

Military avionics has a rough equivalent of OBD built in, called Self-Test. I worked on that stuff for a lot of decades. When I got the Sienna I was scared of OBD, but once I got the scanner and studied some, I realized, hey, I"ve been doing this stuff for decades. Now, I have a better scanner, though on Toyota there isn’t much use for it. Next scanner will include sensor graphing.

Now, really, with my background do you believe I am going to take an intermittent to a $100/hr place to find a problem, knowing what I know? That intermittents are not easy to find? That is expecting too much of an experienced Senior technician.

I knew what the problem was with 95% certainty. Why should I spend a cent for another person with less experience to try to figure it out? Makes no sense to me. If a given mechanic doesn’t like that, we get another one who wants work more than he wants ego strokes.

One of the best mechanics I ever had work on my cars, I refer to his high diagnostic skills, told me he was trying to sell his business. He said the problem was, he didn’t need a mechanic to take it over. He knew very well exactly what I did. He said, “I need someone just like you. Electronics with heavy experience in troubleshooting digital and microprocessors. I can teach anyone to replace parts. But, I can’t teach them to do what you do. And, I can’t pay as much as they pay you guys at the contract defense plant.”

Your attitudes are correct for housewives and beauticians and accountants and ditch diggers. But, high tech Senior technicians, not so much.


Not even if you do?

Rod Knox would be my mechanic if he were available.


I had the same problem and it turned out to be a simple fix. The breather hole in the gas cap was blocked with dust.


Since I am rather ignorant of modern computerized vehicles. When I am having a problem, I try my best to accurately describe what is/is not happening and use correct terminology. I just now thought of a comparison. A race car driver who can furnish their crew chief with that information seems to be more successful than ones who tend to “tell” the CF what they need.


We do not have company mechanics to work on our vehicles so we take them to local garages. I remember one pickup went back 4 time for an issue where it would stall at random and not restart.

The first time they threw a fuel pump at it and claimed it was fixed, then a distributor, then a computer, then finally they fixed the real issue by putting a crank sensor in it.

They would “fix” it, run it around the block and call it good.

This particular shop does not have computerized work orders and I do not believe there is a computer in the shop. They still have the attitude that computers are voodoo and they do not have online access to AllData or any other online manuals because as one of the “mechanics” says " It will be a cold day in hell before I let a frigging computer tell me how to do my job"

We no longer take our vehicles there but that shops attitude is not uncommon around here.

The guys that work there are more worried about listening to their rock and roll music and when its time to start drinking and talking on the phone to their mistresses than doing a good conscientious job.

It is what I refer to as having an alcoholic attitude, these guys can not wait to close up shop at 5pm and head to the local watering hole, That is the main goal of the day. The owner is a drunk and sees fit to only hire his good 'ol boy drinking buddies and their pals.

There are not many repair shops here and only one of them will work on those durn furrin cars.


I had an intermittent no start problem with on 98 Chevy Lumina, turn the key and no clicking of the starter. Fiddle with the key, slam the hood/bang the starter and it would start. After a while, I was able to eliminate the starter as the problem and narrowed it down to the ignition switch. It was a frustrating problem as it would happen infrequently and randomly, could not make it come and go, but I was sure it was the switch. Took it to my local shop and told them the symptoms and asked them to replace the switch. They banged the starter with a broom handle and could get it to start and they insisted that the starter was bad. I could not convince them to just replace the switch so they replaced the starter and when they went to move the car it would not start, so they replaced the switch and when they pulled the old one out it had a loose wire on it, sometimes making contact and sometimes not. They replaced a perfectly good starter, costing me $$$. If some one were to tell me the symptoms I would have thought it was the starter also, but I was a good long standing customer of theirs and usually came in with a good description of the problem and was usually right.


Again, I don’t mean to argue but its a little arrogant I think to believe that anything can be diagnosed without spending more on diagnosis than the car is worth. Riviera 3.8, 500K, intermittent stalling. No codes, power still to pump, and good pump pressure when stalls. Starts right up but can stall again or not. Pump replaced, pump wiring replaced, pump relay replaced, computer swapped, coil and module swapped, crank sensor replaced, cam sensor replaced, air sensor replaced, ground wires cleaned, ignition switch replaced, throttle body cleaned, IAC replaced, fuel pressure regulator replaced, and two shops stumped. Car traded and got $500 trade in. Just not worth it to pay for a couple weeks of diagnostic time trying to stumble on the solution. Maybe EGR but doubt it because no stumble and just like shutting the key off with all power retained. I really don’t care anymore but by the time a mechanic gets my car, its gonna be a lot more than a few hours of diagnostics on expensive equipment with expensive tools.


I recall a local very competent independent automotive repair shop in the late 1960s with a very prominent sign behind the front counter.

Labor $25 per hour.

You watch $50 per hour.

You help $100 per hour.

I am also quite willing to pay for expertise I do not have.


Since you seem to hold me in such low regard, and since you already KNOW what I would do, there’s really no point in saying what I WOULD do

I actually did have a few interesting and thoughtful comments, regarding your Toyota and that evap situation, but I don’t care to share anything now, thanks to you

Nice going



That’s just fine with me . . . I’m not upset that you’d theoretically rather have Rod Knox diagnose and repair your car, versus me

I respect you just fine, I know you’re much more educated and almost smarter than me . . . but you’re not the kind of customer I would want to do business with

Even though the other mechanics on this website may not openly agree with me, I suspect some/many of them share my viewpoint on this

No offense intended to anybody


Sounds like these guys should stick to suspension and brakes, and NOT try to fix anything even remotely electrical/electronic