1JZ throttle loss

majestic47

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#1
Hi guys,

so, I recently bought a 1990 JZA70. Runs great until the engine has been heating up for about 20 minutes, driving on the freeway. Then if I give it much throttle at all, it acts like its hit the rev limit, at about 3-4k rpm. I have to let off the gas in order to get any sort of acceleration to shift to the next gear. Let the car sit, and its fine once it cools back down. I don't even know where to start, especially since the problem only pops up after the engine is hot. There's has an exhaust leak on the uppipe, but that's the only engine related issue that I know of so far.
 

tylerjacobsen85

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#2
It could be unburnt fuel popping in the hot exhaust. I would check/change your spark plugs first. But be careful with the coil pack connectors they will shatter like glass If your not careful. If you do break them you can buy just the connectors from toyota and other places online.
 

hvyman

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#3
Check for codes.

Could have cracked coil packs tho. Pretty common.

Plugs are good to change but coils go bad more often.
 

twin2turbo

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#4
As mentioned check for cracks in the each coil pack along the stems. Also you may have blown capacitors within the ECU, this can cause all sorts of strange issues. If its bad enough the acid can eat through the circuit board rendering it unfixable.

Id also check for codes

Look into a knock sensor rewire aswell. The shielding on the original wiring will be heat damaged and worn out. You can rewire them with RG-58 stranded coax cable that you can find any most wiring stores.


Just a few things to check
 

majestic47

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#5
Changed spark plugs, and the ignition coils don't appear cracked. I got the issue to duplicate yesterday, after getting off the freeway, from going 75mph (120kmh) for about 20 minutes. After I parked, I gave it a little gas, and could watch the RPM's bounce rhythmically. Drove the car again about 20 minutes later in a little slower traffic, for a little longer, and had no issues. I get steady blinks on the check engine light when bridging from TE1 to E1 and starting the car, so no codes.

Is it possible to test the coil packs? I'll obviously want to do it when the problem occurs again. Can I replace just 1 of them? or will I need to replace the whole set?

Thanks for any help.
 
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hvyman

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#6
Pull the blue boots off and check closely.

It will almost look like it’s a plastic mold mark. But really there tiny cracks. There not going to be gaping holes. Just fine cracks.
 

majestic47

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#8
I'm being cheap, but looking to change the coilpacks soon, and also add an adjustable fuel pressure regulator. I changed the fuel filter, since it needed to be done anyway but no fix. Also checked for boost leaks, and I found a tiny one at one of the blow off valves I need to seal off.

The car is still very drivable. The acceleration issue happens worst when I floor it. like my redline moved to around 4,000 rpm. If I dont put the pedal to the floor, its almost like it wants to happen, but doesnt. Hard to take video while driving.

heres a video of what it does in neutral after I experience the acceleration problem. I hold the gas pedal, and the rpm's bounce. let off, and idle is way low. I can duplicate this problem by holding at about 3500 rpm and then letting off slowly.

 

hvyman

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#9
could be bad map sensor. or hose sucking closed.

pics of coil packs with boots off?
 

seoul4korea

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#10
If you google or look on amazon they sell denso coilpacks for a lot cheaper than purchasing them any where else. They are the exact same thing as the OEM 1J / 2J coil packs.
 

majestic47

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#11
I hope I'm not celebrating too soon, but I think I fixed it. Day 2, and I can't get it to repeat.

When I originally posted, I didn't completely know how to induce my RPM's refusing to rise. It turned out not to be heat, the problem was just very intermittant. Playing with it more, I found out that opening the throttle all the way would cause the rev limit at about 4,000 rpm every single time I punched the gas.

I went into my dash to put in some LED's, and also pulled out my speedometer to grease the odometer gears because it was making a funny noise, and when I put it all back together and drove the car, I got a check engine light. Ran for codes, and got code 42, vehicle speed sensor.

I found a few posts about MK4's getting code 42 coupled with the same rev limit symptoms I've been getting, (just topping at a different RPM). I checked the JZA70 wiring diagram, and saw that the speed sensor goes through the speedometer, and then to the ECU.

http://www.supra.co.nz/jza70/tewd/2-28.html

There's screws that make the connection between the speedometer and circuit board on the back of the dash. They were corroded, so I cleaned them with contact cleaner and a brass brush, and gently scuffed up the contact points on the circuit board with scotchbrite.

Now no more code 42, engine revs normally, and I can't get the RPM to bounce anymore. No hesitation anywhere, no matter how hard I try, and the problems were very easy to induce once I figured out how.

I've had a number of problems with this car that have turned out to be corroded or worn contact points.

I also cleaned the throttle body and idle air control valve, which solved my problem of the RPM dropping too low and almost stalling which you can see in the video.
 

Justin727

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#13
keep at it. I know it's aggravating but you'll figure it out soon enough.

For Denso coils I found www.RockAuto.com to be cheapest. Got them for my 1jz back in Dec.

Also have you checked the ecu capacitors by chance?
 

majestic47

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#15
Ok, no more problems with "fuel cut" before red line.

The fuel strainer on my pump was gummed up. Replaced that, and the RPM's now rise normally.

Also had the tank cleaned and sealed.

The ground coming from the fuel pump was cut, so I dont even know how the pump was running. All better now, though.