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93 4runner, back fires through the intake when cold

Old 08-26-2018, 11:56 AM
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93 4runner, back fires through the intake when cold

93 4runner, 4x4, automatic trans, 3.slow engine, 24000 mile on entire engine and drive train. When engine is cold it starts rough and clears up in about a 30 seconds. Then within the first 1/4 mile at the first stop it will pop once through the intake and has a non responsive throttle. After it hits temperature it is running better than it ever has.
So far I have replaced the TPS, adjusted timing after belt replacement, replaced cold start valve, replaced mass air valve, all vacuum lines, brake booster, computer with no change.
I do have one ticking valve, but it is on the passenger side so I am waiting for summer to end to fix.
The three things I have noticed are, the cold start pop, the temp sender has gotten a bit jumpy, or iritic, and when in park and you try to bring up and hold the engine RPM anywhere between 1000 and 2000 RPM it instantly try's to drop to idle, impossible to hold an RPM.
Getting tired of messing with it. Any ideas as to what it could be?
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Old 08-26-2018, 02:37 PM
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https://people.well.com/user/mosk/EFI1.htm
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Old 08-26-2018, 08:34 PM
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Good information although it is for the 22R motors. The 3.0 and 22R FI is very similar and from what I read i seems the fluctuation in the temp sensor could be causing a lean misfire. I will check that tomorrow. O2 sensor may be another possible. O2 was tested a few thousand miles ago, but you never know, so I can check that in a few weeks when I install the new headers, manual trans and locker.
Thanks for the input
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Old 09-06-2018, 10:49 AM
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I have been through just about everything on this system and although it is running the best it ever has; it still has about a 1 second lag on the throttle from a dead stop with an occasional pop through the intake and air box. It gets slightly better if TPS is readjusted, but I have adjusted to the Toyota manual specs and by ear many, many times and still have the issue. I have also replaced the TPS. Nothing on the vehicle other than the body has over 20000 miles on it. The vehicle acts like it needs an acceleration pump. Does anyone have any suggestions?
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Old 09-06-2018, 10:53 AM
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Also I have fixed the serge situation. That was the TPS. I am now not struggling to hold an RPM, but I still have the of idle hesitation with an intake pop
on occasion cold , or warm.
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Old 09-06-2018, 01:18 PM
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It's just a wild guess, but have you tried anything with the cold start ignition injector? It could be sticking open and causing a rich mixture longer than its supposed to. Maybe try pulling the harness off before starting and see how it goes. I completely disabled mine and plugged the injector fittings. I only did it because I broke the line while changing out my head gaskets and needed to get it running as soon as possible. It takes a few extra turns of the engine to get started when the engine is cold, but no other symptoms besides that.
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Old 09-06-2018, 03:29 PM
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cold start.

Yes , I just changed the cold start injector as well just last week. no change. I replaced the brittle plastic line with a copper line and silver soldered it the original fittings so I would not have to worry about it breaking.
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Old 09-06-2018, 03:33 PM
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Today I am trying to work with the throttle valve opening. I am curious if it is closed to tight and when it opens quickly there is lo large a surge of air and momentarily gets a split seconded lean condition causing
a lean misfire. We will see.
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Old 09-06-2018, 06:53 PM
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Finally. All problems gone. The cold start problem was as TheBFA mentioned. I already changed the cold start injector, but as my luck goes, I had two bad ones. I changed it a third time and it starts smooth.
The rough idle I found that the vacuum line had fallen off the power steering idle up valve. Connected with new line and idle is smooth.
The back firing through the intake turned out to be the TPS sensor. I adjusted it one last time and as it turns out if it is turned up all the way to the stop the backfiring went away and my throttle now has no hesitation. It still ain't no LS2 response, but it will do. What a finicky TPS!
Now if it will pass smog control tomorrow then I can get on with some upgrades with New headers and exhaust, a manual transmission; out with the crap A340H automatic, and last but not least an aussi locker in front.
Only one problem left and that is the A/C idle up. It idles down when the A/C is turned on instead of idle up. Been trying with that one for two years and 4 different idle up valves. Just don't get why?
Thanks again guys. Being a chevy mechanic the Toyotas have a definite learning curve for me and they have their quirks!!!
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