I have a 1985 Toyota Pickup, 50k miles (yes, only 50k), manual, 22R engine. Starts and idles fine. When engine is cold, it will run and accelerate smoothly for a few minutes if not moving, or for about 1/4 to 1/2 mile if driven. Then, as it starts to warm up, problem with acceleration begins. When not moving (just revving engine) it will accelerate smoothly at first, but then when I reach a certain RPM range, the hesitation/bogging down/running rough begins. Don't have a tachometer to measure RPM, but problem seems to begin at about 2000 RPM. It's whatever the RPM range is for "normal" driving. When starting off in first gear, problem begins as soon as clutch is released, but smoothes out as accelerated up to about 20 mph to shift into 2nd. Clutch has to be released very slowly when starting off, and have to rev higher than I like to keep from stalling out. In second gear (about 20 to 30 mph) problem not as bad as coming out of first, possibly because engine RPMs are now lower. When in 3rd, (between 30 to 45 mph), problem still evident. Once in 4th problem not as bad, again maybe due to lower RPMs. I have looked for anything in Repair Manual that might be temperature related and have checked: Automatic Choke (heater resistance of 22 ohms within spec), Choke Breaker (disconnected vacuum hose and applied vacuum, and choke valve opened slightly), Choke Opener System (checked BVSV with cold engine, and checked BVSV, diaphragm and linkage with warm engine), Auxiliary Acceleration Pump (gas did not spurt out from acceleration nozzle when pinched hose released after engine stopped, and diaphragm checked OK when vacuum applied and released), and Deceleration Fuel Cut System (fuel cut solenoid valve "clicks" when tested, and 0 resistance when vacuum switch tested). Have been told it might be the fuel pressure or distributor. Took off fuel pump and it tested OK. Relatively new plugs (3k miles) fuel filter (1k miles) Have checked plugs, wires, distributor advance OK, coil tests OK. Haven’t been able to test ignitor under load as none of my local auto parts stores seem to have plug adaptor to be able to connect to test machine, and the instructions in my Repair Manual for testing seem contradictory, so I haven’t tried those either. With air cleaner removed and all associated emissions control tubing, problem still there, so not HAI or MC related. Whatever the cause, it is definitely temperature related. Anyone have similar problem (and more importantly) suggestions or even better, solution? Thanks!

 

Update: Think I've narrowed it down to EGR system. Inner-most filter in EGR Vacuum Modulator has dark burnt-looking spot where air is suppoed to pass thru filter. Tested BVSV valve by blowing air into pipes when both cold and hot, and it checked OK (closed or no air passage when cold, and open or easy air passage when hot). Checked the EGR Vacuum Modulator by disconnecting P, Q, and R hoses, blocking P and R ports with fingers, and trying to blow air into Q port. Air did not freely pass thru to the air filter side, but it did pass, and could be felt if filters removed. If I disconnect EITHER the Q or P hose when engine is hot, it has none of the hesitation problems, but will knock or ping very badly. Disconnecting just the R hose has no affect and still have acceleration hesitation. Have not taken off EGR Valve itself yet to check for sticking or carbon deposits, as it looks to be a lot harder to get off. Now waiting for engine to cool off to see how it runs when cold. Can anyone tell me if the EGR Vacuum Modulator is the culprit based on these results, or it is still possibly the EGR Valve itself? Since engine starts and runs OK at idle, does that mean EGR valve is seating properly?

 

Update #2: Took off EGR valve, found some carbon build-up at entrance to intake manifold. Scraped and spray cleaned EGR valve with carb cleaner. Still hesitates when hot. Have re-checked system with vacuum gauge and all seems to be as it should: Have low vacuum pressure (3 in) going to EGR valve when hot and reved up to normal driving RPM. Have zero pressure when hot and just idling. Have tested just vacuum modulator with vacuum pump, and vent to atmosphere appears to be working correctly: Can pump or pull air thru Q at idle when P & R closed off with finger; cannot pump or pull air thru when reved up to 2-3k rpm. If I hold vacuum with hand pump and slowly let off throttle, decrease in exhaust backpressure to modulator allows air to enter thru modulator and I lose vacuum. When applying vaccum just to EGR valve when engine is off it holds until vacuum is released, and can hear diaphram closing with "thump". Stumbling and hesitation still seem to indicate I am getting too much EGR flow ... next step is to take off EGR valve again and try cleaning again, and check for valve sticking open. When I had it off first time stem and diaphram seemed to move freely when vacuum was applied. Disconnecting the EGR system and thereby eliminating all hesitation seems to nail problem as being with this system, but am baffled by all parts of this system appearing to work as they should. Anyone have any other ideas what could be causing too much EGR flow?

 

Update #3 (and final): Replaced EGR vacuum modulator and solved my problem.

 

THANK YOU for posting your results. Finally searching through the countless threads without solutions and I find this!!!