1990 Supra Turbo, stock.
It has a miss off idle. It idles perfectly smooth, and it seems to pull smooth under load, but it has a miss when driving at a steady speed with light to moderate throttle. This problem has worsened over time.
This miss is also very apparent when free reving the engine, it starts to miss just off idle, and appears worst at about 2000 rpm. (The engine actually shakes at around 2000 rpm when free reving it.)
What I've done so far:
Replaced every vacuum line that looked at all suspect, replaced accordion hose, replaced all PCV hoses, replaced all Bypass Valve hoses, cleaned throttle body and PCV orifice, set up TPS, checked valve clearances, checked coil pack resistances, checked O2 sensor as per TSRM, checked timing, (10 degrees) checked cam timing, new NGK 8mm wires, (loomed nicely in clips, instead of laying in the valley like spaghetti) new NGK BCPR6EGP plugs. Probably some other stuff I'm forgetting at the moment.
All this has improved the problem, but not gotten rid of it.
I had a tough time with the TPS, but eventually got it set, however the resistance change is not linear, the resistance jumps around as you move the throttle, rather than steadily rising to infinity. Is this normal?
The O2 sensor made from 9 to 12 cross counts in ten seconds (I tested several times) but it was erratic, it would cross count a few times, then pause a couple of seconds, then cross-count several more times rapidly.
Is this normal?
The old plugs were very dark, not quite black and sooty, but quite dark brown. They were BCPR5EP, one range hot, which makes me think the PO might have been having issues with it.
For lack of other ideas I pulled the PCV hose off, creating a large leak of unmetered air. The miss disappeared like magic.
So my question is, where is the next logical place to look for the problem?
Edit: sorry, should have mentioned, no codes, none.
Edit II: another detail that slipped my mind, this happens only with the engine at operating temperature, for the first few minutes until it warms up, no miss.
Solution: Two problems.
One: the tube from the EGR to the vacuum modulator was plugged with carbon. This may have meant that when the exhaust pressure signal got to the modulator it tended to keep it pressurized. In any case I cleaned it out nicely with carb cleaner and a wire.
Two: more of the idiot PO's work. The modulator was plumbed backwards. The modulator has a dump built into it with a little filter that should be cleaned occasionally. The filter is under a snap-off snap-on cap on the valve. The cap has the port labels, "P" & "Q" on it. The cap will snap back onto the body either way, but the labels only be right one way. See where I'm going with this? Result, the engine tended to suck a little unmetered air from time to time, but more importantly the EGR could not dump it's vacuum properly.
I took the EGR valve off to do the cleaning, not something I would recommend to anyone, unless you have hands and arms much smaller than mine and like having your patience tested.
It has a miss off idle. It idles perfectly smooth, and it seems to pull smooth under load, but it has a miss when driving at a steady speed with light to moderate throttle. This problem has worsened over time.
This miss is also very apparent when free reving the engine, it starts to miss just off idle, and appears worst at about 2000 rpm. (The engine actually shakes at around 2000 rpm when free reving it.)
What I've done so far:
Replaced every vacuum line that looked at all suspect, replaced accordion hose, replaced all PCV hoses, replaced all Bypass Valve hoses, cleaned throttle body and PCV orifice, set up TPS, checked valve clearances, checked coil pack resistances, checked O2 sensor as per TSRM, checked timing, (10 degrees) checked cam timing, new NGK 8mm wires, (loomed nicely in clips, instead of laying in the valley like spaghetti) new NGK BCPR6EGP plugs. Probably some other stuff I'm forgetting at the moment.
All this has improved the problem, but not gotten rid of it.
I had a tough time with the TPS, but eventually got it set, however the resistance change is not linear, the resistance jumps around as you move the throttle, rather than steadily rising to infinity. Is this normal?
The O2 sensor made from 9 to 12 cross counts in ten seconds (I tested several times) but it was erratic, it would cross count a few times, then pause a couple of seconds, then cross-count several more times rapidly.
Is this normal?
The old plugs were very dark, not quite black and sooty, but quite dark brown. They were BCPR5EP, one range hot, which makes me think the PO might have been having issues with it.
For lack of other ideas I pulled the PCV hose off, creating a large leak of unmetered air. The miss disappeared like magic.
So my question is, where is the next logical place to look for the problem?
Edit: sorry, should have mentioned, no codes, none.
Edit II: another detail that slipped my mind, this happens only with the engine at operating temperature, for the first few minutes until it warms up, no miss.
Solution: Two problems.
One: the tube from the EGR to the vacuum modulator was plugged with carbon. This may have meant that when the exhaust pressure signal got to the modulator it tended to keep it pressurized. In any case I cleaned it out nicely with carb cleaner and a wire.
Two: more of the idiot PO's work. The modulator was plumbed backwards. The modulator has a dump built into it with a little filter that should be cleaned occasionally. The filter is under a snap-off snap-on cap on the valve. The cap has the port labels, "P" & "Q" on it. The cap will snap back onto the body either way, but the labels only be right one way. See where I'm going with this? Result, the engine tended to suck a little unmetered air from time to time, but more importantly the EGR could not dump it's vacuum properly.
I took the EGR valve off to do the cleaning, not something I would recommend to anyone, unless you have hands and arms much smaller than mine and like having your patience tested.
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