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Discussion Starter · #1 ·
Okay so I've spent a ton of time trying to sort this issue so hopefully you all can help me come to a conclusion.

Whenever driving my car in excess of 5 minutes it starts to hesitate and I dont mean like creeping up to speed it jerks as i press the accelerator. (imagine you push in the accelerator and let of and on and off causing the car to rock as im moving forward) I can send a video to anyone if thats not clear enough.

What I've replaced

Fuel Pump

Fuel Filter

Spark plugs with the correct gap

Dizzy Cap and Rotor

Now remember this isnt as soon as i start the car, I can drive it however I want. Then 5 minutes into my drive it bucks no matter how I am on the accelerator.

If I need to explain further let me know.

Cheers
 

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Sounds like a boost leak. Lots of threads about this. Check all of your intake piping for leaks. If you have an aftermarket blow off valve, that is another source. You can build a cheap device to check for these leaks as well. Again, do some searching. Google is your friend.
 

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This also sounds like the TPS is defective or out of adjustment. The TPS should trigger a fuel cut when it reports throttle closed at driving speed, and if it reports this when the throttle isn't actually closed then you get a sharp buck. The 5 minutes driving time may be the time it takes the engine to warm up enough to activate this feature.
 

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Discussion Starter · #4 ·
Sounds like a boost leak. Lots of threads about this. Check all of your intake piping for leaks. If you have an aftermarket blow off valve, that is another source. You can build a cheap device to check for these leaks as well. Again, do some searching. Google is your friend.

sorry guys 1987 MK1 N/A
 

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The first few minutes of operation the symptoms probably don't manifest due to the cold start timer is keeping the injector open. How's the condition of your intake piping(assuming you are running the AFM and not MAP system. If the clamps are loose or the piping is cracked the AFM "flap" could be making big movements. If your motor mounts are weak or shot it would really amplify the condition. I had a cracked lower elbow on one of my cars and it had a similar problem. Another thing to check would be your vacuum lines, as the engine moves a larger vacuum line could leak enough air to cause that condition. Again, not as noticeable while the car is in 'cold mode'. Just a couple of things I've experienced. Good luck.
 

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The cold start injector is supposed to close the moment that you release the starter. It does have a limiting timer, but that should only come into play after extensive cranking.
 

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I had this same exact issue, although on a '91 N/A. Turned out to be a bad O2 sensor. I swapped it out with a new one and it hasn't had the issue since. Good luck!
 

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When this happened to me before, it was fixed by replacing the spark plugs. But it looks like you've done that already.

Interestingly, in my case, nothing looked obviously wrong with the old spark plugs that were causing the problem, except that replacing them fixed the issue.
 

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Sorry as this may be off topic but even with the issues the MR2 has that seem to be an endless and bottomless pit of issues know that it's still better than the Supra in my opinion. In fact Lux Velocity just did a write up on me Charto and the comparison of the Supra TT versus the MR2.
 

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Try unplugging the TPS. The car should run acceptably without it, and that would allow you to eliminate it as a cause.
 

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Does it buck under light gentle throttle or 1/2 throttle or more. I had an issue years ago with my '85. Under light throttle it was fine. When I hit it to the floor it would buck like a cylinder cut out. Weirdest thing. It turned out one of the injector plugs was broken. Under light throttle the engine wouldn't rock and it was fine. under heavy throttle the engine would move and that was enough to cause the plug to lose contact with the injector effectively killing the cylinder. I found it accidentally while messing with the throttle in the engine bay I happened to bump the # inject wires and it stumbled. zthe plastic was cracked on the plug part of the harness and the lil wire clip wouldn't hold it in place. I ended up changing all 4 plugs. The plugs are different on the 85/86 than the 87-89s though. Worth a look.
 

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^ Common situation. On the 85-86 cars the plug latches are extremely fragile, and they are almost always broken. I think that the plugs are better on the later cars.
 

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you are correct Rmeller. When I installed the smallport into my '86 I used the '86 ecu and what not but switched to the later style injectors I had from another engine and the smallport connectors.
 

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Discussion Starter · #15 ·
Does it buck under light gentle throttle or 1/2 throttle or more. I had an issue years ago with my '85. Under light throttle it was fine. When I hit it to the floor it would buck like a cylinder cut out. Weirdest thing. It turned out one of the injector plugs was broken. Under light throttle the engine wouldn't rock and it was fine. under heavy throttle the engine would move and that was enough to cause the plug to lose contact with the injector effectively killing the cylinder. I found it accidentally while messing with the throttle in the engine bay I happened to bump the # inject wires and it stumbled. zthe plastic was cracked on the plug part of the harness and the lil wire clip wouldn't hold it in place. I ended up changing all 4 plugs. The plugs are different on the 85/86 than the 87-89s though. Worth a look.
I will give this a look when I get home however I think all my connectors are solid.
 

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Discussion Starter · #17 ·
Update---


I unplugged the o2 sensor the other night and drove it right after and it continued having the same issue. So I let it sit for a couple days and took it out last night and it drove like a champ (o2 still unplugged). Drove it this morning like I literally beat the hell out of it for about a 5-6 mile trip and it didnt miss a beat. So my money is on the o2 sensor at this point.
 

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Try replacing it and see. Never had a bad O2 sensor do that. Usually when they go bad the engine just runs really rich. After replacing a bad O2 sensor my mpg went from 24 mpg to 30 mpg.
 

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Discussion Starter · #19 ·
Try replacing it and see. Never had a bad O2 sensor do that. Usually when they go bad the engine just runs really rich. After replacing a bad O2 sensor my mpg went from 24 mpg to 30 mpg.

Drove it for about 50 miles round trip just now, didnt miss a beat. That is with the o2 unplugged. Will continue testing
 
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