QuestionMy 1994 Corolla wagon (1.8 liter California car w/manual tranny) has 230k freeway miles, but has had regular maintenance and TLC. It gets 30-35 mpg and uses no more than 1/3 qt oil per 1000 mi. It passed smog two years ago with average HC & CO readings and well below avg NO readings. I am mechanically inclined (have rebuilt Nissan, Ford, VW, Toyota motors) and reasonably well equipped to do repairs (no oscilloscope, tho).
Recently, 30-minutes up the freeway toward Lake Tahoe, the engine started stumbling and hesitating as if it was processing some bad gas (elevation 3500 ft). The problem got worse until it seemed to be stumbling around on only 2 to 3 good cylinders under any load, and even at idle. This continued downhill all the way home, but has been somewhat intermittent since - it starts fine and sometimes drives a few miles before the idle sputtering (and stumbling under load) begins again (but with inconsistent severity).
Ignition components checked OK (strong spark): plugs, rotor, and cap were replaced 10K miles ago, wires are spiral-core (<1.5k ohms resistance). I replaced the coil on a hunch that it was changing resistance with heat - no luck.
Several sensors tested marginal so I replaced them (TPS, ECT). - No luck. The IAT checks OK. The upstream and downstream O2 sensors were replaced 5k ago. Following the check procedure outlined in Haynes, I get 9-10 voltage oscillations per 10 sec at the diagnostic VF terminal with the T and E1 shorted, which tells me at least one sensor is functioning (not sure which). Checking the VF learned value voltage (T open) reads 2.1 volts, suggesting the mix is a little rich (?) The fuel pressure gauge reads 46 psi without vacuum and 37.5-38 psi with idle vac (22" Hg), i.e., 1-2 psi higher than the specifications tolerance. Vacuum is strong, so perhaps the fuel pressure is consistent with the VF learned value. I blew the return line and it felt OK. The driveability/idle problem persists.
I cleaned the EGR modulator and checked the EGR valve by manually applying vacuum to it - Haynes manual says that with the engine running at idle, I should be able to stall or nearly stall the motor, but I can only get vacuum after jumping the T and E1 terminals to trigger the VSV switch, which allows me to stall the engine. I conclude that the EGR and VSV are working fine.
Timing is dead on 10 degrees with the T and E1 shorted. When not shorted, the timing wavers between 5 and 7 degrees. A friend with a Camry says this is normal. The only other strange thing is that shortly after jumping the T and E1 connectors, the idle drops dramatically (still w/10 degrees timing) and it sounds like the EGR is re-circulating gasses. I don't understand how this can happen because the E or R ports have no vacuum at idle, and the throttle stop dashpot is functioning.
With the MAP disconnected, the car seems to run very sluggish and rich (smelly) but without the hesitation. Back-probed MAP voltage is 4.95 volts signal, 3.4 volts output at 0 vac, and 1.1 volts at 24" Hg vacuum (manual pump limit) - the high-vac reading is a bit high according to Toyota on-line service advice, but Haynes says 0.5 to 1.5 volts is OK.
Any ideas? Should I replace the (expensive) MAP and/or fuel pressure regulator?
AnswerThis sounds a little complicated but I will try to have some sort of answer for you, everything sounds normal except for the fact that you are not able to make the engine stall by applying vacuum directly to the EGR valve without using the jumper wire, to me there is no doubt that the egr system is being activated when it should not be, that is, at idle or just off idle, the drop in idle speed when jumping the T and E1 terminals is normal, try replacing the EGR VSV to see if it helps. The only other thing I can think of is that there may be some vacuum hoses that are connected incorrectly, check them against a vacuum diagram, if you don't have one let me know.
