I am working on a 1984 Lincoln Town Car with EEV-IV throttle body injection. The car is entirely stock and extremely well taken care of, with all original systems on the car (air injection, EGR, cats). Apparently the engine was replaced 10,000 miles ago for some reason with a Vege Reman DF46 (same as stock as far as I can tell).
The problem is that during warmup, and only then, the engine will buck and misfire for maybe a minute or two until reaching temperature. So, to be clear, it runs fine cold and warm, but does this only for a short period of time in between.
The problem disappears when disconnecting the EGRV/EGRC control solenoids - so disabling the EGR appears to "resolve" the issue. We have scoped out EGR vacuum vs the EVP (Egr feedback) sensor, and the computer is in control of EGR - the valve moves linearly in relation to vacuum. There are no deadspots or discontinuities in the feedback signal. A visual check of the EGR valve yielded nothing abnormal and it has full travel and seals ok; passages are not plugged.
Next we checked exhaust backpressure via an in-cylinder pressure test on each bank. The results are similar between both banks, and backpressure appears to be spiking to roughly 10-12 psi during rev-up and 3 psi at idle. That seemed excessive (although I don't know if it is on this setup), so be drilled inspection holes front and back of each bank's catalytic converter; everything looks pristine. There is nothing blocking it and no collapsed material visible. The muffler is new, so this also doesn't appear to be a factor. Of course, there may be collapsed material inside the secondary air injection section of the cats, but how can I check for that without drilling the cat apart? I am reluctant to sell a pair of new cats that do not solve the problem at all.
Also, the computer code readout yielded the following:
CO Codes: none
KOEO Codes: none
KOER Codes: 21 (ECT out of range).
So, of course we also checked the ECT. The wiring is continuous and the sensor behaves linearly and within range (checked at 5 different temperatures against the chart). Replacing the ECT with a resistance equal to about 190F makes the computer happy. I think the issue is that a lower temp thermostat might be installed (heat gun reads ~180F at thermostat, factory is 195*F). This makes me believe replacing the ECT will not resolve this and is not a factor here.
Is is possible that somehow the thermactor air injection system is injecting too much into the manifold (via the TAD diverter valve)? I'm not sure the pump could output enough air to affect backpressures though; and this would also show up during the KOER test.
Some other notes:
- Throttle body has clean bores
- Problem appears gets worse with lower outside temp (preheat stove pipes and vacuum are connected and check ok)
- Problem is 100% repeatable when starting with cold engine
My next step would be two secondary ignition pickups; one on cyl. 1 and one on the coil wire to see if it is always the same cylinder misfiring.
What would your suggestions be on where to go from here?
