Diagnose Engine Stumble / Rough Patch in 3rd gear from 1800-2200 rpms (1 Viewer)

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Hi all,

Would like some help diagnosing or at least theorizing possible causes for an engine stumble / patch of rough running which lasts from 1800-2200 (approximately) rpms only in 3rd gear. This issue has been going on for several years.

I have an H55f transmission and the original 3FE. I've done the usual items such as spark plugs, wires, rotor, cap, O2 sensors, rebuilt fuel injectors, new fuel filter. No check engine codes. MAF has been replaced with a new (used) unit. TPS (new oem) has also been replaced and calibrated- although perhaps I should check it again. FPR was replaced with a new unit, but it was defective--so the old one was put back in. Valves have been adjusted.

In all other gears and RPM ranges, the engine runs smoothly. However, it is noticeably rough in the exact situation described above. Any ideas, comments on general theoretical causes, or thoughts are appreciated.

Thanks.
 
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When the valves were adjusted could you or whoever did it, could the amount of carbon build up be seen on the valves and possibly tops of cylinders? I ask because my valves were just adjusted by a reputable Cruiser specialty shop recently, and the tech commented on this. In response to this I just had a significant sweep of engine and fuel system cleanings that the Justice Brothers provides in their "professional service products" line; the 3-step Emissions System Cleaner and the Fuel Injection System Service. The Fuel System Service is done by disconnecting the fuel line to the injector rail so that the chemical can clean that entire assembly and the injectors. Given my 3FE has almost 240K miles, I didn't expect a hot rod, but where I do notice it is in second and third gear accelerations (auto tranny). If my throttle position (foot) is such that second gear shifts at 2200 RPM to 2500 RPM, there is now a bit stronger power in third gear. Dramatic, no. Feel better, yes.

Heads up on protecting any paint or powder coated surface from the chemicals in this pro service line. They will damage those surfaces if spilled/dripped on them.

For whatever it's worth, that was my experience.

Have you done a leak down test in recent history? That may tell you something useful. Mine are still below the 25% Toyota spec, but it is a "well seasoned" engine.
 
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I adjusted the valves. There was a moderate amount of carbon build up in the valve train but about what is normal for a an original 3FE with several prior owners and no service history. I think its gotten cleaner gradually with more frequent oil changes.

I removed the injectors and had them professionally cleaned. Also, I desmogged the engine. Both improved power and engine response but the problem I have described remained.

Haven't done a leakdown test, but the last compression numbers several years ago were 150, 148, 150, 150, 139, 151.
 
To rule out whether or not it's the EGR valve, disconnect it's vacuum hose and give it a test drive. But you can't permanently run a 3FE without the EGR functional without other not-good ignition problems cropping up. But at least you'll ge able to rule it out
 
To rule out whether or not it's the EGR valve, disconnect it's vacuum hose and give it a test drive. But you can't permanently run a 3FE without the EGR functional without other not-good ignition problems cropping up. But at least you'll ge able to rule it out

Engine has been desmogged. I only have still have catalytic converters.
 

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