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Vintage 164 question

S

ssholland

New member
Joined
Jun 13, 2015
Why do Volvo 164 runs better when the air intake temperature sensor#1 is disconnected? When I connect it, the engine slows down considerably and acts like it might stall. When I unplug it the engine perks up and runs better. I have checked the sensor and it is within ohm specs.
 
ssholland said:
Why do Volvo 164 runs better when the air intake temperature sensor#1 is disconnected?
Click to expand...

Function - To enrich mixture when air temp is below 68?F; apparently, via disconnection, the analog computer detects cold air, and enriches mixture.

Consequently,

1. Engine may or may not be running lean...vacuum leak
2. Fuel injection pressure is not up to spec
3. Engine temp sensor might be off spec
4. Manifold vacuum (pressure) sensor may be off spec
5. Vehicle speed sensor is inop or off spec
6. Auxiliary Air Valve not working correctly

Manual here
 
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84b23f,

Thanks for your reply. I plan to diagnose all that I can. With that in mind, I wanted to know your opinion about a couple of things. First, I tested the ECU yesterday and here are a few readings. The pressure sensor ecu harness pin reading 7 to 15 was 88.4 ohms. I think spec is 90 ohms. The pin reading 8 to 10 was 336.0 ohms. I think spec is 350 ohms. Do you have any thoughts on these readings? Are these readings close enough or should I replace the MPC? Thanks.

Steve
 
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By the way, what is a good CO2 sampling device? My engine does run rich. I can smell it strongly.
 
One more by the way.... after I repaired three vacuum leaks, the a/c (I don't really have a/c, just the fan) fan longer cuts off during heavy acceleration. I assume all leaks have been repaired.
 
NO_SPRK said:
if you need it i have a un-modified 164 map sensor "grenade" for sale.
Click to expand...

Thanks. I may take you up on your offer. Can you confirm given the specifications that I provided is proof I need to replace the MPC? Thanks again.

Steve
 
ssholland said:
Do you have any thoughts on these readings? Are these readings close enough or should I replace the MPC?
Click to expand...
Given specs are not absolute, they always have some differences. I'd say your measurements are close enough.
 
I will check the fuel pressure tomorrow. Cold start injector does not drip. Also, I checked the air temperature sensor, and it no where near spec. I checked it recently, or so I thought. I have ordered a replacement. This could be the fix. One more, I directly checked the MPC, and the 7-15 readings was exactly 90 ohms and 8-10 pin reading was 350 ohms.
 
The fuel pressure is actually supposed to be 30 psi. This Service Bulletin was issued to correct the service manuals.

SB27.3%20B20_30%20E_F.jpg
 
ssholland said:
I will check the fuel pressure tomorrow. Cold start injector does not drip. Also, I checked the air temperature sensor, and it no where near spec. I checked it recently, or so I thought. I have ordered a replacement. This could be the fix. One more, I directly checked the MPC, and the 7-15 readings was exactly 90 ohms and 8-10 pin reading was 350 ohms.
Click to expand...

The most common failure for the MPS is a broken diaphragm.

To diagnose, check to see if the MPS holds vacuum.

A broken diaphragm will make the engine run rich all the time.

FYI, a broken diaphragm can be replaced.
 
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84B23F said:
Starting Points:

1. Rail Pressure...is it at spec value...28 psi

2. Cold Start Injector...is it dripping?
Click to expand...

I have not tried verifying the fuel pressure or checked the timing, HOWEVER, when I was troubleshooting yesterday, I came across the thermostat time switch wiring was off. A certain mechanic had bound the positive and negative wires on to the same connector. I spliced and wired for separate connectors. I started the car wayyyyyy easier today. Not perfect, but 70% better than before. It warmed up nicely and has a bit of a saw in the idle-below 1000 rpms. Nothing big. I hope with the addition of a replacement air temperature sensor soon, since the old was shot, I expect a better ride.
 
ssholland said:
few readings...MPC...
Click to expand...

Manifold Pressure Controlled fuel injection system - This sensor works as follows.

Idle - Minimum amount of fuel injected.

Full Throttle - Maximum amounts of fuel injected, as based upon a given RPM.

So, from idle to full throttle, vacuum decreases within intake manifold. High vacuum at idle, and low vacuum at full throttle.

Vacuum hose to this sensor must be fit for duty, and intake manifold must not be sucking extra air in the area where this sensor gets its vacuum. Extra air entering intake manifold where this sensor sucks will increase fuel amounts, which means a richer mixture.
========================

Auxiliary Air Valve (If equipped) - If one exists on this vehicle, it too must be operational...if it is not closing when reaching 95?F, then a higher idle will exist, unless someone twitched idle adjustment.
==============

Thermostat time switch - I assume this is hooked to cold start injector. All it does is provide enrichment fuel during cold temps, for some given period of time.
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Saw in the idle-below 1000 rpms - It could be related to fuel/air, electrical, defective engine valve, or uneven compression in all cylinders.

Considerations:

1. See if injector O-rings exist, and renew all of them, after testing them and cold-start spray patterns. Old o-rings tend to allow extra air into intake (I assume injectors are in intake manifold...never seen this setup).

2. Throttle Valve - If its shaft has free-play, it will suck air. All high mileage vehicles will have shaft free-play. A carburetor shop might be able to re-bush, but I would not be concerned at this point.

3. Throttle valve - It must be set to their spec...when done, this may assist in determining what is wrong. If someone has opened it wider, then extra air enters intake chamber, and this will increase fuel enrichment.

4. Throttle valve switch - Adjustment is done after throttle valve is set to spec.

===========

Needless to say, all vacuum hoses must be fit for duty, and air valve hose's hoses too.

Again, fuel pump pressure must be at spec...if possible, watch fuel pressure when engine is running.
 
The IAT sensor is correcting the fuel mix for the change in air density at different ambient air temperatures. The IAT sensor has a negative slope meaning that the resistance drops as the air temperature increases. Disconnecting the IAT essentially makes the IAT sensor resistance infinite which the D jet controller is interpreting as a very low ambient temperature with very dense air and is increasing the fuel PW to account for the higher air mass. In the absence of very dense air, this will cause the engine to run rich which may account for the smell of the exhaust. There may be other problems which the low AFR is masking which is why it may drive better. When my B20E had a broken IAT sensor connection, it did not drive better.

You said your IAT is no where near spec. If the resistance was much lower than spec then that would cause the controller to shorten the fuel PW resulting in lean running. Disconnecting the IAT would cause the opposite effect which may be better than super lean operation.

By idle sawing do you mean that the engine is speeding up and down? If so, there is a chance that the throttle switch is incorrectly set or there is a problem with one of the contacts in the switch. The D jet system is sensitive to the correct set up of the throttle switch. Check the operation of all the contacts as per the service manual and follow the procedure set out in the manual for setting idle and never use the throttle stop screw to try and set idle.

Your resistance values for the MAP sensor seem reasonable. As noted, the MAP sensor transformer wiring rarely fails, its the diaphragm mechanism that fails. Using a Mighty vac or equivalent, if the MAP sensor holds a steady vacuum of -7 psi (0 being atmospheric) the diaphragm is probably not leaking. If the car drives reasonably well its probably safe to say that the MAP sensor is functioning. Speed X Density EFI systems don't work very well / at all with out a functioning MAP sensor. If the diaphragm is broken or otherwise leaking slightly the MAP sensor is going to read atmospheric or close to atmospheric pressure and the D jet controller is going to interpret that as wide open throttle and is going to increase the fuel pulse width like crazy which will, as noted, cause it to run super rich. I have never experienced it; but, I expect a broken diaphragm or severe leak would make the car un driveable. A slight leak may allow operation - that is why the leak test with a vacuum tester is important.

Unlike modern cars which lengthen the injector pulse width during cranking to provide enough fuel for starting, the D jet does not do that. The D jet relies on the external cold start injector for that extra starting fuel and if it was disabled starting in cold temperatures would be exceedingly difficult bordering on impossible where I live.

With respect to manifold air leaks, fix them before you try and set idle. Manifold air leaks on a speed X density system don't result in fuel mix changes. They are just like more air flow through the aux air valve and result in higher idle speed. Common leak locations are the seals on the injectors and the shaft on the throttle plate.
 
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Ok, I thought I would close this out with the latest. No vacuum leaks, fuel pressure 30 psi, timing spot on, did rewire the thermal time switch, replaced the intake air thermostat and the biggest was adjusting the co2. It has never idle so well before. It now has a very minute hesitation during acceleration, but I believe a minor adjustment to the throttle position sensor will resolve this. Thanks for all the input and the incredible amount of useful information. Amazing.
 
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