Back-pressure test? Never heard of it. I did put my hand over the end of the exhaust to see how much back-pressure. When on idle it is not very much. I was going to remove the 2 bolts holding the converter on and see how it runs like that. Should not be to hard to do.
you unscrew the oxygen sensor and screw the gauge in instead, to take your reading
Granted, tight valves will cause lowered compression but the odds of that on all 4 cylinders make it unlikely.
A wet compression test will answer that question PDQ without a valve adjustment.
I’m of the opinion that if a major engine problem exists, which I feel is the case here, fretting over anything else is irrelevant.
what kind of gauge? Is this a special oxygen sensor?
“all 4 cylinders”
This car has a straight 6
6 cylinder. Renting a oxygen sensor and comp. tester today.
M bad; I forgot about the Supra part of it…
Six cylinders just makes it even more unlikely that tight valves are the cause of the problem as the odds of that on all 6 are pretty slim.
The poor and comparatively good running could be due to fluctuating fuel pressure; among other things. It’s also possible that the “good” running is not as good as one might thing. It just seems that way after running like crap.
It will be interesting to see what the results of a wet test turns out to be. Supras were pretty strong runners and a lot of them got flogged pretty hard.
A Nissan dealer I worked for took some of them in trade back in the day and every one of them had been rode hard and put away wet; as the dealer discovered later.
Did fuel pressure test. Was on the lower side of the value the book says, but it was very steady.
Tight valve lash might be the reason for low compression across the board
A tired engine might also be the reason . . . in that case, it would be bottom end, I would think
Yeah, the wet compression test result would be interesting
Recently, I worked on a tired old 4 banger Saturn
It had low compression, just like OP’s Supra. All around 100, I believe
On 3 of the cylinders the wet compression test results were the same, meaning a valve job might be in order
On 1 of the cylinders, the wet compression was 180, indicating a bottom end problem for that cylinder
Even though the car had low compression, it was running smoothly.
How do the black spark plugs correlate with pressure test?
110 to 130 is not super low. I do not think that would effect if running so poorly.
I’ll just bow out of this. I guarantee you that I know of what I speak. If you choose to flog a dead horse then so be it.
Nobody said you don’t know your stuff
But maybe we enjoy flogging the dead horse
The compression values you report are a little on the low side. I’d expect to be seeing 150+ in all cylinders. 130-140 on some cylinders an older engine maybe. But that measurement can result in inaccurate results. For example if the starter motor isn’t turning the engine robustly for some reason, whether the engine is warm or cold, position of throttle & choke plates, whether all the spark plugs are out or not, accuracy of the gauge, how many cycles you waited, etc. Suggest you explain how you did the measurement.
The fact that connecting - presumably – disconnected ground wires back to the chassis makes the engine run worse, that leads to maybe a problem with the ignition system. Given the shards of metal in the distributor you found, I think it is a good idea to at least pull the distributor and see how it looks on the bench. Maybe it has too much play in it.
The typical way to test the EGR, this is how I do it on both my 90’s Corolla and my 70’s truck, is to use a hand held vacuum pump, put a vacuum on it while the engine is idling. A proper operating EGR valve, once the vacuum is applied, will cause the engine to promptly stall out.
Tested compression. lowest was 105 highest 130.
Tested vacuum. was very steady at about 19 1/2 psi.
Tested fuel pressure. was steady at 26 - 27, which is on the low side from what the book says, but should be ok.
Found 2 wires that were not connected. One was a ground to the side of the intake, just above the block. Believe it to be a ground for the temperature gauge inside car. Gauge did not work before, now it does. Car is not over heating by the way.
Other wire went to the vicinity of the electronic coil.
Since I hooked both of these wires up it has ran worse. Tried adjusting the timing, no timing light, but would not run very decent.
Checked ERG valve by removing and looking over, seemed not to be much vacuum there, but seems to be working fine. Blew air from a air compressor into it and could see the rubber part move, so assume it is good.
Car idles ok, it will idle perfect for a couple minutes and then back fire for a half minute or so and then idle perfect again. Has little power to move. When going down the road, sometimes it will run a little better if gas pedal is not to floor, but a quarter way down.
I may drop the exhaust before the cat converter to see if that does anything, which I doubt it will. It does have a little back pressure out the muffler, but not a whole lot.
I may also swop distributors that i have from a parts car of the exact same year. By the way, I did notice tiny metal deposits inside the distributor. Cleaned it out. Any idea what that is about?
Seems like it is running out of time. Any other suggestions on what to try?
I was going to do the wet compression test. I was going to do a dry then a wet. Perhaps I had a poor compression tester from Autozone last time. This time I get from Oreilies, and the test was much higher. I will give the readings from first to last in order. Each I took at least 3 tests.
1 - 140, 145, 150.
2 - 140, 145, 145.
3 - 150, 155, 155.
4 - 140, 140, 145.
5 - 120, 130, 135, 135.
6 - 120, 120, 120, 125.
Since the last two were lower, I did a wet test on them. These were the readings.
5 - 150, 150, 150. And again when not wet. 130, 135, 135.
6 - 120, 120, 125. And again when not wet. 120, 120, 120.
According to the Haynes book, the minimum low end is 120 something.
All though the last couple, last one at least does concern me, I do not think my problems are not a totally worn out engine. Tired, perhaps, but not worn out totally shot. Even if it was actually worn out, I do not think it would run the way it currently has. And it does seem to be very moody, where one day it will run better then another.