Mk1 2ZZ swap that used to work fine now won't rev above 5k?

Labcoatguy

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Posted this on the Spyderchat forums too, but figured worth a shot here if there's any secret Toyota tuning experts around.

Situation: 1988 Mk1 MR2 with a 2ZZ swap from previous owner, has been running fine for about two years, kicking into lift like whoa as expected. All stock electronics with the factory Denso MAF in the intake piping and an O2 sensor installed after the header merge.

This summer, it started to seriously lose power and bog down on hot days, occasionally even stalling when stopped. Here's what I did next:

  • Suspected knocking, added octane booster to the next fillup and cleaned the MAF with MAF cleaner spray, and after putting it back in, that's when it stopped revving above 5k as if a rev limiter kicked in at that RPM.
  • Later found that the MAF mount had separated from the intake tube (JB weld failure), fixed that, but not before panic-buying a new MAF sensor and crank position sensor.
  • Installed them, still wouldn't rev above 5k, and got P0171 P0325 codes (might've been left over from beforehand).
  • Disconnected the battery to clear codes, went out for another drive, doesn't bog down as much but still can't rev above 5k.
  • Read the codes again and the only thing that came up was an evap system code, which I'd expect, as the guy's swap didn't include that.
I wonder if the ECU just needs to cycle a few times before letting itself get above 5k again. Anyone know what might be behind this?
 

Spectre

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I do not know if this applies to the 2ZZ engine but there have been a few Japanese engines that refuse to rev above a certain RPM when they lose their cylinder head/coolant temperature sensor for the engine management system. The Nissan VG30s are notorious for it - they seem to run fine except they just don't rev above (IIRC) 2.5-3K. Maybe check connectivity to the sensor and test the sensor itself?
 

Labcoatguy

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I was wondering if it'd be some sort of stealth sensor failure like that. I'd put the oxygen sensor up as a suspect too, as my gas mileage is piss-poor (<15 MPG) even taking into account the fact that I drive it pretty aggressively.

EDIT: Hmm, it's neither of those; coolant temp sensor is in spec, as is the O2 sensor heating circuit. I'm getting a heater circuit code for the rear O2 sensor but I don't have or need that. It may just be a vacuum leak somewhere along the intake, which would be supremely annoying.

EDIT 2:
Someone on Youtube seems to have the same issue.
Claims in the comments it's a camshaft position sensor.

EDIT 3: Camshaft position sensor is within resistance spec. Back to square 1.
 
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Labcoatguy

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Update: my bad, I didn't install the new crank position sensor right. The bolt wasn't tight enough and the ground connector wasn't making contact with the engine. It works now.
 

Labcoatguy

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Maybe it was an old crank position sensor that was giving me unrealistically high readings before; now it's cutting off at the expected 8k-ish RPM.
 
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