M/T Failing? Don't know how to determine . . . - Mazda MX-6 Forum
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post #1 of 29 (permalink) Old 6-18-19, 23:22 Thread Starter
 
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M/T Failing? Don't know how to determine . . .

Hello Everyone~! For the most part my Engine seems to be running pretty well. It has a slight sputtering at idle, but I have been through every system I can think of. My timing belt skipped 17 teeth two weeks ago, so I did a timing belt/water pump job, and the noisy engine thing is minimal now ('93 MX-3 1.8L V6 with the noisy exhaust cam washer thing). My primary problem is a seemingly HUGE loss of Horsepower. Poor acceleration, down 30-50% I would say. On the expressway, full foot to floor will get me a top of about 81 mph, and, with any slight incline, my speed drops down from that, so, I can barely maintain Highway speeds. Car has 265,000 miles on it, and owes me nothing, but I do need to figure out my main problem. My neighbor suggests a possible failing Manual Transmission. I can see his point; if the M/T is failing, there could be tons of extra friction within it robbing all the lost horsepowers. Thing is this: I have never experienced this kind of failure. Shifting is as good as it has always been for me, owning the car for the last 30k miles. Clutch is fine; nothing is behaving differently about the transmission, other than possible increased internal friction of some sort. Has anyone here had anything like this happen? Also, I have kept close tabs on my cylinder compression. All cylinders remain at + - 200 psi, still, even with so many miles on it. (By the way, the timing belt that came off looked totally OK, but I now understand that we have suspect automatic tensioners. The idlers were slightly worn, but, not enough to skip the T belt). I am currently trying to ratchet up my plans to do a JDM Engine and Transmission Swap because of my big issue. Whatever it might be, it is significant enough to worsen and strand me (again). Any input would be highly welcomed, and thanks in advance.
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post #2 of 29 (permalink) Old 6-20-19, 16:02
 
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I've been on this forum (and the 626 and Probe forums) for many years, have read thousands of threads, and I don't remember anyone reporting that sort of problem with the manual transmission. And it would take a lot of internal friction (generating a lot of heat, and almost certainly other symptoms) to take away that much power. So I think that's very unlikely.

You are sure it's not a slipping clutch (rpm goes up but mph doesn't)? OK.

I'm not sure about the 1.8 V6, but isn't 200psi compression actually higher than it's supposed to be, unless some previous owner installed a high compression ratio jdm version? But anyway, we can rule out slipped timing belt.

My hunch would be fuel or spark. 265,000 miles is a lot. Blocked fuel filter? Weak/failing distributor or plug wires?

One thing that is unclear from your post: did this loss of power happen immediately after you replaced the timing belt? Or is it unrelated? Edit: if the loss of power only happened *after* you replaced the timing belt, and the 200psi compression was measured *before* you replaced the timing belt, then I would strongly suspect valve timing -- you put the timing belt on a tooth or two off.

Last edited by NickR; 6-20-19 at 16:08.
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post #3 of 29 (permalink) Old 6-20-19, 16:56 Thread Starter
 
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I didn't think it through that well, Yes, the friction would create tons of heat if the Tranny was the culprit, and if it got hot (I have driven it like this for several two hour drives) I think it would not shift properly; it shifts fine and no, the clutch is not slipping. Now, before I did the timing belt job, I checked compression in four cylinders, and they were all down to 120 psi. That is what convinced me to go ahead with that job. Now that the timing is correct, I probably should do it again just to verify improved compression. Both O2 sensors are very new; Bosch brand (which I invested in after crappy results from eBay cheapo's_). The cat is basically new too, and I did take it off a couple weeks ago to verify not the problem, as that was suggested by two different people. Oil consumption hasn't been that bad, and has even improved since I started using AT 205 Reseal, but I can't give you numbers on miles / quart. Engine uses no coolant whatsoever. Fuel economy is definitely down. On the highway, I used to get 28 mpg going fast with the A/C on (in Arizona no less); now probably down to 23 mpg if I were to estimate, and I can't use the A/C basically at all right now as I just don't have enough power. I had read that possible issues could be the coolant temp sensors, but, I would think no because the lost power shows up immediately after cold starts. I would like to say that the onset of this problem was pretty sudden too (and before the Timing Belt skipped 17 Teeth). I was on the expressway, and maybe needing more foot into it than normal without realizing it, but when I switched on the A/C I began to lose speed, even as I mashed the peddle to try to keep it up. Had to turn off the air to get home, and the problem has been this way ever since. I will go out and check my compression right now. . . . Updated Compression readings: 1=205psi, 2=175, 3=205, 4=185, 5=205, 6=210. Other things I did include New Coil, New Fuel Pump (my neighbor's gauge gave me a false low reading; pump was fine), Recently changed all vacuum solenoid valves and vacuum hoses, and since I did get a TPS code for some reason, I tested and re-calibrated that.
Oh yeah, I did a complete Timing Belt job; the whole kit; all new pulleys/idlers, tensioner, and water pump. Also, I have the JP50 MAF; that thing will never go bad. So, someone else suggested possible O2 sensor issue even if I get no code for them. I realize they can be bad and get no code; the ECM can't tell if it is getting bad signals. So, the next thing I will do is try to figure out how to test the O2 sensors.
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post #4 of 29 (permalink) Old 6-20-19, 18:15
 
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Given the new compression numbers, I think we can rule out any problems with the timing belt/valve timing.

The bad fuel economy measures suggest it's not a problem with fuel filter. And bad O2 sensors might cause slightly worse fuel economy, but I don't think they would cause that big a power drop.

My current hunch: a bad engine coolant temperature sensor. It's running rich, because the ecu thinks the engine is cold. I have heard a few cases of this happening, even without the check engine light throwing a code (you have checked for codes?). So you get low power and bad fuel economy. I suggest disconnecting it, as a test, just to see if it improves performance.

Otherwise, it's maybe a problem with the distributor?
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post #5 of 29 (permalink) Old 6-20-19, 19:27 Thread Starter
 
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I do have two of the three coolant sensors sitting around (and the third, if I can figure out where I hid it from myself). Now, if I recall right, I disconnected one of them and ECM gave me an immediate code, which disappeared soon as I hooked it back up. BTW, disty is very new, and just last week I tossed in a new coil I had for good measure. Fuel filter is new, so is pump, and pressure regulator. I might install the new sensors very soon. Thanks for checking back in on me, it's good to have a partner in crime;-)
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post #6 of 29 (permalink) Old 6-20-19, 20:49
 
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If you simply disconnect the main (middle) ECTS, the cel comes on (code 9), and both fans come on high (if I remember correctly, because the 93 is different), but the ecu goes into default mode and the car should run fine, with full power (except maybe difficulty with cold starts). So if you can't find the new sensor, that's an easy way to test my hunch.
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post #7 of 29 (permalink) Old 6-20-19, 23:36 Thread Starter
 
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Oh, cool. I should go try that now, because I had planned on swapping the sensors (I found all three now) first thing in the morning, but want to do it on a cold engine. Also, I recorded the resistance of each new coolant sensor, to compare to the ones I pull. Honestly, I think it is a long-shot for a solution, but, I'm open to being surprised, and, beggars can't be choosers!
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post #8 of 29 (permalink) Old 6-20-19, 23:55 Thread Starter
 
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HOLY CRAP! I think that might have been the trick! I didn't know which one you meant, so, I just pulled all three connectors. I think it started up better, and idled better, immediately. Now, I only went around the block, but I DO think I had the acceleration back. I have been running with the TEN/Grnd bridged anyway, and earlier today I got back my EGR code 16. Then, I had three codes. 16, 9 (like you said), and 5. I will do the job in the morning, then, later I will take it out on an expressway errand I need to do. Wow, I feel a great burden being lifted off my shoulders right now. Very refreshing! I'm still a bit skeptical because the reasoning isn't registering with me yet, but, hopefully by the end of tomorrow afternoon I will be a very happy skeptic. Thanks for your attention Nick!!! And for not giving up on the project.
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post #9 of 29 (permalink) Old 6-21-19, 5:32
 
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I'm keeping my fingers crossed, but that sounds like great news! Let us know how it goes.

The 93 MX6 (I'm not sure about the MX3, but it's probably the same) has 3 coolant temperature sensors:

1. The "front" one, next to the coolant filler neck, is a 2-wire sensor that only controls the fans, and gives code 69. (94 and later models deleted this sensor, using the "middle" one to control the fans too.)

2. The "middle" (or main) one, next to the timing belt cover, near the end of the fuel injector rails, is a 2-wire sensor that controls fuel/air mixture and cold idle speed, and gives code 9.

3. The "rear" one, immediately beside and to the rear of the "middle" one, is a smaller 1-wire sensor that controls the temperature gauge on the dash, and gives no code.

I remember a few other people with MX6, 626, or Probes, having similar symptoms to yours, that turned out to be the middle temp sensor.

BTW, a large spark plug socket will roughly fit that sensor, if you don't have a proper long socket to remove it.

BTW, here is a write-up on replacing the knock sensor without removing the intake (to get rid of your code 5): https://mazda626.net/topic/31675-how...-knock-sensor/
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post #10 of 29 (permalink) Old 6-21-19, 19:17 Thread Starter
 
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OMG, I can't believe I thought my car ran better going around the block last night. It is running worse than ever now. I just got back from a lousy interstate jaunt, complete with stop/go traffic. I had the coolant sensors plugged in on the way there, and I disconnected them for the way back; no difference. Just, bad bad bad. And yes, I think I was getting code 69 on my way there, so, I don't think my ECM likes the new middle/main (green) sensor. I measured resistance of the new ones before I installed them, and the resistance of the old ones, before I threw them out. Vastly different readings, in fact. I do have Denso O2 sensors on the way now. I kinda doubt there is anything wrong with the very recent Bosch ones in there now, but, it appears that throwing parts at the problem is what I am left with doing.
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post #11 of 29 (permalink) Old 6-22-19, 4:42
 
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Damn! That is very disappointing.

Going back to your original theory: does the car coast normally in neutral (or if you put the clutch down)? If "yes", I think we can rule out anything non-engine related. Because binding transmission (or brakes) would make it slow down much quicker than normal.

Spark plug wires, or distributor cap or rotor? And just checking (because we want to check anything at this stage) the spark plug wires aren't crossed (because, as you maybe know, the order of wires on the disty cap is not the same as the 123456 firing order, and it varies across disty caps, because of a cross-over of wires inside the disty cap, so you must check the numbers printed on the disty cap itself).
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post #12 of 29 (permalink) Old 6-22-19, 7:29 Thread Starter
 
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Yeah, wires, spark plugs, cap n' rotor are all good/recent. Firing order is correct, I made that mistake before with the firing order, so I know what you speak of. And, car does coast fine in neutral; no brake, wheel bearing, or transmission problem. I will plug the coolant sensors back in this morning while the engine is cool, and see if it still registers Code 69.
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post #13 of 29 (permalink) Old 6-24-19, 6:46 Thread Starter
 
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Yep, code 69; new sensor ordered (again).
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post #14 of 29 (permalink) Old 6-24-19, 19:30
 
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I remember reading on one of the forums that some parts stores have the wrong part number listed for the front sensor (the one that controls the fans, and gives code 69). But the only problem if you have the wrong part for that front sensor is that the radiator fans come on too early; it won't cause the engine itself to run badly, like yours.

Just a hunch: you don't have the wires swapped to the front and middle sensors?
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post #15 of 29 (permalink) Old 6-25-19, 1:47 Thread Starter
 
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Hmmm, that's an interesting hunch. I will have to get back to you on that tomorrow. If I can't tell, then I will swap them and see what happens.
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