Okay Bricksters - I'm getting close on this one. I'm really focusing on the driveability issue with severe hesitation/bucking at low speeds.

Since my '90 740 GL has the Regina FI system, hence no AMM, it uses the manifold air pressure sensor and the intake air temp sensor to derive the same information. Using a regulated vacuum pump and backprobing the MAP sensor yielded very strange results. The Chilton's chart depicts the 5vdc reference signal increasing from 0.5vdc at 2.9psi to 4.4vdc at 14.5psi. My results moved in the opposite direction!

Voltage Voltage
PSI Chilton Measured
--- ------- --------
2.9 0.5 3.99
5.8 1.1 3.44
8.7 2.1 2.96
11.6 3.2 2.44
14.5 4.4 1.88

Before I spend $133 for a new MAP sensor, is this fairly refuteable evidence of a blown sensor? The vacuum line to the sensor was in great shape, cleaned and inspected for leaks anyway - also cleaned vacuum port on the manifold even though it appeared to be fine. Already cleaned TB, adjusted TPS.

Two additional questions:

(1) Chilton's book states the IAT sensor voltage should vary with the temperature of the intake air. Unfortunately, there's no chart in the manual. Does anyone have this information? And...how would I take an accurate reading of the intake air temp to reference against anyway? Volvo dealer service tech couldn't find it on their on-line system - waiting for a call-back.

(2) I don't own an o'scope so I hooked my DVM to the O2 sensor signal lead and monitored the voltage swing during a test drive. Infomation from a certified Volvo mechanic was that optimal, average voltage range would be around 450mV to 500mV. W/O a scope, he said to look for voltage swings from 100/200mV to 800/900mV. Albeit, the following is an "eye-ball" of the voltage ranges, this is what I observed:

fully warmed idle 0mV-1100mV (seems to be wide swing from lean to rich)
during severe hesitation 30mV-50mV (very lean - very steady)
(<30 mph)
30 mph + 200mV-900mV (somewhat within spec)


So, since an extremely lean mixture condition seems exists during the hesitation problem, does this still point to the MAP sensor and the backwards readings? Higher vacuum = higher speed...should = higher MAP voltage to ECU and more fuel? FI relay does not cutout during hesitation or higher speeds, RSR relay was swapped with fan relay which lessened the cold start problem but hasn't affected the hesitation problem. FPR maybe?

Tommy
1990 740 GL
B230F/Regina FI/170K