Dramatic Powerloss and Temp Pegged to Red

[K1CKBAC]

Hey guys,

Been trying to diagnose this issue for the past few days. The car would start buckling, temp gauge would go all the way to red, and the car would struggle to get up to speed. I'm thinking it would be a grounding issue since the temp gauge is the one that is affected, but not sure which ground point it would be. The oil pan to chassis ground cable is new. Possible that the battery is on its way out?

PS: Not sure of this is related but the radio antenna sometimes would roll up and sometime would not.

Car is a 1989 325 stock.

[mark///m3]

Is the car actually overheating when this happens?

[jlevie]

Does the gauge go instantly to the red when the engine misbehaves? If that is what happens I'd suspect a problem in the engine harness.

[K1CKBAC]

It goes instantly to red around the same time it starts buckling. When I turn on my headlights, the temp gauge would twitch as well. I would then turn off the car and wait a bit to turn on the car. Afterwards it runs fine.

The engine isn't overheating. I would turn off the car and turn the keys to accessory and the temp reads normally.

[jlevie]

That really sounds like it may be a bad engine harness. Other than the electrical system the harness is the only thing the engine management system and the temperature gauge have in common. And even in the case of an electrical power problem (loss of power) the gauge is almost certainly going to drive to full cold. I suppose it is remotely possible for a voltage spike to affect both the gauge and DME, but the gauge has a bias coil in it to make it essentially immune to variations is system voltage.

[K1CKBAC]

I tried to diagnose the problem today but it got dark however I recorded the problem and uploaded to youtube. I notice it starts happening when the car starts to warm up.

http://youtu.be/W67hwW2Rwgk

[jlevie]

That is an electrical problem. Since I don't see the tachometer or econometer reacting, I'm thinking a wiring problem involving the temperature sensor and the injectors. You might be able to see it with a noid light (check both banks).

[K1CKBAC]

Thanks for the input jlevie. What is weird is when I attempted to start it again, same thing happened but the econometer was also jumping.

[jlevie]

If the engine was in open loop mode (below 140F) when you shot the video, data from the O2 sensor wasn't being used by the DME. But if the engine was hot enough to be in closed loop mode this time, the DME would see the engine AFR change and it would react in an attempt to make it right. Which would cause the econometer to react.

[K1CKBAC]

I'll definitely look into that. I was on my way to work and the car would not start. Does not seem to have spark or fuel or both. Can a bad oxygen sensor do that?

[jlevie]

I'll definitely look into that. I was on my way to work and the car would not start. Does not seem to have spark or fuel or both. Can a bad oxygen sensor do that?

No, but a bad main relay, bad CPS, bad ignition switch, bad fusible link, wiring fault, or a bad DME are all possibilities. Check the simple stuff, but with the other problems you have the harness may be at the root of both problems.

[K1CKBAC]

Thanks, I'll look into the harness over the next few days.

[jlevie]

You may find this helpful...

For the engine to run the following conditions must be met:

Power on DME pins:
27 Start Input
18 Un-switched Power input
37 Power Input from Main Relay

Ground on DME pins 2, 14, 19, 24

Timing data from the CPS on DME pins 47 & 48 from a rotating engine

To have spark power must be present at the coil positive and ground pulses
from the DME's pin 1 must reach the coil negative. Power to the coil is
controlled by the ignition switch via C101. When checking for spark, use the
output lead from the coil to eliminate the distributor, rotor and plug wires.

To have injector firing power must be present at each injector and ground
pulses from the DME's pin 16 (Bank1) and pin 17 (Bank2) must reach the
respective injector bank. Note that the injectors are wired as two banks of
three. With cylinder 1,3,5 being bank 1 and 2,4,6 being bank 2. Power to the
injectors is controlled by the main relay. Injector firing is best checked
with a noid light.

The fuel pump relay must have power on pin 86 (relay coil) from the main relay
output (pin 87) and power on pin 30. The DME will ground pin 85 to turn on the
relay and power the pump(s) via pin 87. Of the above, only the fuel pump power
is fused. So if the there's power at pin 87, but not at the pump, check fuse
11.

The main relay and DME pin 18 receive power from the smaller of the two wires
that connect to the battery's positive terminal. That wire incorporates an
in-line fuse. When the DME is presented with a start signal, it grounds the
main relay pin 85 and furnishes power to the fuel pump relay, injectors, and
DME.

Troubleshooting:

Disconnect the battery and the DME cable. Then:

1) Disconnect the coil negative and check continuity from that connector to
DME pin 1. Also verify that from DME pin 1 to ground is an open circuit.

2) Check the resistance across DME 47 & 48, which should be 500-560 ohms. If
the CPS is dismounted, the resistance can be seen to change from about 500 to
540-560 ohms when a ferrous object is brought to the face of the
sensor. Neither pin should be grounded.

3) Check for continuity from DME 36 to main relay 85 and from DME 3 to fuel
pump relay 85.

Reconnect the coil, remount the CPS (air gap should be 1mm), plug the
relays back in, reconnect the DME, and connect the battery. Then do the
following checks:

1) With the key off, verify that power is present at DME pin 18 and main relay
86 & 30.

2) With the key on, verify that power is present at DME pin 27 and pin
18. Power to pin 18 is from the main relay and there should be power to the
injectors and fuel pump relay.

3) With the key on, verify that no voltage is present at the DME grounds (2,
14, 19, 24).

4) Verify that power is present at the coil positive and at fuel pump relay
pin 30. Those get switched power from the ignition switch via C101.

The engine will start and run (if poorly) with only those connections to the
DME in place. The other signals from Cylinder ID, AFM, temp sensor, etc., are
necessary for proper operation. But they won't prevent the engine from firing.

IMPORTANT:

A power check means seeing a voltage within about a tenth of a volt of what
you measure across the battery terminals, which should be at least 12.6v on a
charged battery.

A continuity check means seeing less that 1 ohm of resistance.

An open circuit means seeing a resistance of at least 100k ohms.

A good quality auto-ranging digital multimeter will make these tests much
easier.

[K1CKBAC]

Update: I changed the ground wire that goes from oil pan to chassis and it started right up! Even though the wire is only a few months old, for some reason it was not grounding anymore.

[jlevie]

The engine ground strap only matters for the starter, alternator, oil pressure light, and temperature gauge. All of the engine management system parts have wired grounds back to the DME. So I don't see how fixing the ground would affect the operation of the engine. I suspect the problem is still present.