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1985 Toyota 22R with hesitation ONLY when hot

Experiencing hesitation/misfiring/bogging down sensation under light to moderate acceleration. Manual transmission, and sometimes feels like bucking by a novice stick-shift driver. Starts fine, idles fine when cold or hot. Will accelerate fine when cold, but only for a few minutes or less than a mile until it’s warmed up enough. Have checked and tested many ignition and carburetor related items until narrowing it down to EGR system, which is only operating once warmed up, and not at idle, so that part of the system is working OK. If I disconnect vaccum hose from EGR valve, runs fine with no hesitation, but pings or knocks badly (as expected). Symptoms all point to too much EGR flow. With no computer or codes to check, I haven’t found too much help on on-line forums for problems like this. Have tested both EGR valve and EGR Vacuum Modualtor, and both seem fine. EGR valve hold vaccum. Found and cleaned some carbon build-up, and pintle stem doesn’t seem to stick. Vacuum modulator tested by blowing air into Q port while blocking P & R and air passes thru small bleed off hole. Filter had small dark burnt looking spot at bleed off hole - don’t know if that’s normal or not. Also tested modulator by simulataneously apply pressure to bottom port (to simulate exhaust pressure) while applying pressure to Q port, with P & R still blocked. Bleed off hole slowly stopped flowing air as pressure built up. About to roll the dice and replace modulator (less expensive of the two), unless someone has better opinion or suggestion of what it might be. Without trying to add more confusion, have wondered if a clogged cat convertor would cause enough increase in exhaust pressure to make modulator send too high a vacuum signal to valve. While the truck is old, it only has 50k miles, and no real modifications. Any ideas?

The EGR valve should be closed during acceleration…Most engines will run FINE with the valve disconnected.

“If I disconnect vaccum hose from EGR valve, runs fine with no hesitation, but pings or knocks badly (as expected). Symptoms all point to too much EGR flow.”

If it pings or knocks, check your ignition timing and the advance mechanisms. The ONLY time the EGR should be open is steady-state cruising. it is closed at idle or during acceleration…

Also, it might be time for a carb rebuild, a job never to be taken lightly on these complex Japanese feedback carburetors…If the power circuit is not enriching the mixture during acceleration, some of the symptoms you mentioned can be expected…

Thanks for the other suggestions, but considering that when engine is stone cold it runs and accelerates fine, would timing still be a suspect? I did check for vacuum advance on the distributor and that seemed to work fine, but have not yet specifically checked timing.

Perhaps a better way to describe the knocking would be that when engine is warm to hot and EGR system is disconnected, knocking under driving condition or load will occur, in proportion to how hard I accelerate. With a very light foot, hardly pings at all. When in neutral and just reving engine, no pinging or hesitation. Much of what I’ve read about EGR indicates that too low of a flow will cause pinging, and too much will cause hesitation.

A carb rebuild is well beyond my capabilities, so I am hoping it is not that … it’s taken me a week just to read, re-read, and re-re-read FSM and get a grasp on how the EGR system is supposed to work!

The engine should run fine under all conditions with the valve disconnected. When there is acceleration demand, the valve is normally closed to provide maximum power. If you are experiencing spark knock, look for the cause elsewhere… Overly advanced timing is one cause and a lean fuel mixture is another…

The fact that it will accelerate smoothly when cold (no EGR, choke on) points to a lean mixture…Dirty carburetor, non-functioning power valve or other mixture control device…But check the timing first, since that’s the easiest, most basic adjustment. With the light connected, you can verify that both the vacuum and centrifugal advance mechanisms are working correctly…

Update: Replaced EGR vacuum modulator and all is well. While it seemed to pass all of the diagnostic tests, it apparently wasn’t responding accurately enough to changes in exhaust backpressure to send correct amount of vacuum (or at correct time) to EGR valve.

I had the same problem. The vacuum line was off what i believe to be the Vacuum Advance