740 Missing, Idle Hunt, Occassional Stalling

[ncomstock]

Hi Friends, long time lurker but first post. Thanks for solving so many of my past problems with the search bar.

Recently brought into the fold a 1990 740 N/A with auto trans and Regina fuel system. The car has unknown service history, but was obviously taken care of because of the shape its in.

Symptom is intermittent hesitation coming off idle, idle sputter and hunt. Always idles smooth when cold, gets much worse after a half hour of driving and/or starting again after sitting and cooling. Idle hunt is much worse while in gear, and sputters a little during initial acceleration and then clears up at higher engine speed. *Addendum* The surging gets progressively worse and worse while at a stop light and in gear, often ending in a stall. There is a cloud of vapor that comes out of the air box when it stalls like this. Sometimes smells like stale fuel.

Actions taken so far are replacing crank position sensor, replace oil separator system, solve a few vacc leaks, clean throttle body and IAC, replace air filter, reflow solder joints on fuel pump relay to be sure they are connected properly, clean electrical grounds in the engine bay. The previous owner replaced the distributor cap & rotor, wires, plugs. I have a new IAC on order as the gunk really didn't completely come out of this one.

Whats the next move?

*Edit* Continuing testing, vac pressure is between 19 and 21 psi, jumping around a little. Seems to be running rich based on exhaust steam. I would like to know how the coolant temp sensor works, is it possible that the computer is receiving a cold engine temp while the gauge is reading normal?

*Edit 2* The next culprits to investigate will be the fuel pressure regulator (vac lines do not smell like gas), engine temp sensor (gauge reads correctly), radio suppression relay. In the past I have had success with cleaning the contacts on relays for starters, fuel pumps, etc. Is the RSR a similar thing? May respond favorably to being cleaned, or should just be replaced?

[ncomstock]

A half day of diagnosis today did not turn up anything more. I would like a sanity check on something though.

The google search for MAP sensor testing turned up this paragraph:

Regina Manifold Air Pressure (MAP) Sensor. The Regina fuel injection system uses air pressure and temperature in the intake system to compute the air mass moving into the engine. The MAP pressure sensor is located on the driver's strut tower support. It is connected to the intake manifold through a hose and takes a reading of intake pressure. The sensor is piezo-electric and changes a voltage input to an output signal proportional to manifold pressure. The system calibrates atmospheric pressure when the engine is started and under full load conditions. To test, turn ignition "on" and test the back of the sensor connector or the pins on the back of the ECU connector (do not disconnect with the ignition "on"):

Connector A or pin 6 is ground
Connector C is the constant voltage input from pin 7 on the ECU and should read 5 volts to ground
Connector B is the variable output signal from the pressure sensor to pin 11 on the ECU. Connect a hand vacuum pump to B or 11, apply vacuum, and read the signal. Voltage should drop from 5 volts to a lower number. If it does not drop, replace the sensor.

Problems may also occur in the sensor, the vacuum tube leading to it, the connector, or the wiring leads.

The way my car is wired, pin 11/connector C is the 5V supplied by the ECU and pin 7/connector B is the output signal of the MAP, and apply vacc with the ignition on causes the voltage on pin 7 to fall. When I reverse pins B and C at the MAP to jive with the wording of this paragraph, I get 5V at connector B and 3.4V at connector C, and neither of the voltages fluctuate when I apply vacc. Is the paragraph incorrect or have I found a very odd problem?

[volvolugnut]

Suggestions:
This sounds to me like a vacuum leak. If so, the MAF will not be measuring all air into the engine and results will be lean condition. Check vacuum hoses for creaks. They do not age well. Try a water spray on hoses and intake manifold gaskets. The water will temporarily seal the gap and idle will smooth. This method located a leaking manifold gasket and solved my rough idle problem on 245.
Also, variable vacuum reading (by the way, the reading is in Inches of Mercury) is an indicator of a vacuum leak.

If the car ran ok until this started, I don't think it is electrical problem.

volvolugnut

[ncomstock]

Seem to have solved this problem with a new ignition coil. Tested the old one using this method:

Good Coil Test:

I tested the primary resistance (between the positive and negative posts) and on my good coil it read .001 ohms, essentially nothing.

I tested the secondary resistance between any of the positive or negative posts and the lead to the distributor cap wire and it did not register at all, no electrical connectivity at all, no reading on the digital multimeter

Bad Coil Test:

Primary resistance was still .001 but the secondary resistance was 1.6 ohms when it should have not registered anything at all.

The old one on this car measured 1.2kohms, and the chassis of the coil was all rusty. Hope this helps someone else in the future.