I have an 89,240DL,auto,145k, with LH-2.4 . I've been getting fault codes 231,113,221. I removed and cleaned the TB (dirty), cleaned and oiled the ACV, cleaned and greased electrical connectors,tested the resistance in the AMM wire and the OXS and replaced the FPR . Ran well-no codes for a few weeks until today. I beleive the car goes into limp mode(more like criple mode) and will stall if I don't keep the RPM's up. I changed the FRP after lurking here and thought I had it solved. I haven't got to the ECU system electrical tests in the Bentley Book. Are they conclusive? Could the AMM be bad if it passes a resistance test? Thank you for your help. -Jack

Finally did the LH-2.4 System Electrical Tests in the Bentley Book. Found two faults. The 'engine rpm signal' read about 3-4 volts instead of the 8v min. stated and the 'oxygen sensor wiring harness' did not register continuity to ground. The wire from the ECU connector to the OXS plug tested fine. The OXS voltage output "jumps around" more than "flucuates" and loosening the oil filler cap makes no noticeable difference. Plan on buying a new OXS - and I'll test for continuity from green wire to ground for curiosity. What generates the engine rpm signal and would a lower voltage affect the way the car runs?

Jack Popham-Western NY 89-Blue 245 auto 145k, 88-Gray 245 5spd 250k

Jack, the readings don't help much at either point. The oxygen sensor output, when in closed loop mode is alternating between roughly .2 and .85 volts once or even twice a second if in good shape and warm. If you have a digital meter, and depending on the make and model of the meter, you may see nothing but jumbled numbers or occasionally repeated values representing the low and high. This effect is even worse with the rpm signal from the ignition computer, because the rate of change is much higher, it will depend entirely on the luck of your meter's manufacture if you can make any sense of it.

A scope is really needed to see the rpm signal. I think it is also the best tool for diagnosing an oxygen sensor too, because not only do you see the limits of excursion as it crosses the lean/rich point, but the rate of change and duty cycle or ratio of time in rich and time in lean.

I think Fitz makes a good point where he suggests the tests we can make on the oxygen sensor are not conclusive - especially if being done with a digital multimeter. The trick of leaning it out by lifting the oil dipstick or removing the cap, or enriching it by removing and plugging the regulator vacuum are readily compensated by a functioning ECU, so unless it is on the edge of closed loop, you probably won't see any difference on a DMM.

I bite my tongue while suggesting it, but try the universal 3-wire if you feel comfortable about crimping and heat shrink insulation, or the made-to-fit if a smooth quick install is worth money.



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Art Benstein near Baltimore

Aside from the items already mentioned, I have 2 additional things for you to check:

1. Idle Air Control Valve. This is the bypass valve that makes sure that your engine is getting enough air when at idle and at low RPMs. If this is clogged or the motor inside it is burned out, you will have Terrible mixture problems below 2,500 RPMs.

2. O2 sensor. Even though it may show a valid voltage, it can still be the problem. They can slowly die on you and cause the car to run rich -which in turn causes a 'sudo-correct' reading from the O2 sensor (but the mixture is significantly enrichened). Read the following from a previous poster last week:
http://www.brickboard.com/RWD/index.htm?id=618018

Aside from the above mentioned items, I'd guess the Air Filter, Fuel Pressure Regulator, and the Air Mass Meter. All of which can look or test good, but can be off by a little or a lot.

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 214K

I think the AMM will be fine if it's close to spec. Definitely worry if it's outside of specs. These later ones don't seem to be as fragile as the earlier types though. (You should have a Bosch 016 in that car.)
Before suspecting the AMM, I'd look very closely at the pins for it in the harness connector. They do open up and sometimes push back in the connector, making bad contact. That seems more likely to me than a failure of the AMM though I guess it could lead to a failure of it.

DO you know that it is too rich? The engine temp sensor could be bad and that would definitely cause a very rich condition.
That and the fuel pressure regulator are the likely items to cause rich running.

The other thing I'd definitely look for is vacuum leaks- the stalling is definitely a symptom of that. Typically vacuum leaks cause an excessively lean condition, but it could be that simple. My 86 just needed a new intake air hose in order to run right. Holes in it causing vacuum leaks.
Good luck!
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Rob Bareiss, New London CT ::: '86 244DL- 215K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K

The resistance checks are next to useless to prove an AMM good. No strike that, they are useless. Your best approach after replacing the FPR is to be sure the ECT is OK (Bentley resistance method works well for this item) then move back to the oxygen sensor. Make sure the heater works by checking the heater's resistance immediately after disconnecting it. Then get a voltmeter on the signal lead and watch it from cold into warm up. I think you are correct, the ecu is in cripple mode which gets you about 12MPG, and your oxygen sensor is the only feedback the ecu has as to what mixture is run. Start with a clean ECU by unplugging it momentarily. Because of the AMM burnoff timer circuit, don't unplug the ECU until the ignition key has been in your pocket for at least one minute.
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Art Benstein near Baltimore