1995 Geo Metro engine mis

[91-93-96Geoman]

Hello.
I have a 1995 Geo Metro 3-cylinder that has recently developed an engine miss. Primarily on start up when cold, but sometimes at freeway speeds of up to 65-70.
I replaced the distributor rotor and cap, spark plugs and wires.
One time it was surging up and down after startup cold, so I checked and cleaned the EGR valve and cleaned the PCV and it seemed to help somewhat.
When the miss occurs, the check engine light will flash-on for a split-second, then turn off. I think I read some where that I can drive the car with an OBD reader attached and see if it can catch the elusive P-code.
Like I mention, this is a recent problem. The engine runs perfect most of the time. I suspect some type of valve or sensor that is intermittently opening and closing.
Any ideas?
thanks.

[Johnny Mullet]

I highly recommend a compression test................

In order to determine if a rough running engine has internal issues, then a compression test is highly recommended. This is a very simple test and you can usually rent or borrow a compression tester to get your readings. You could also purchase a tester fairly cheap if you shop around. Here is what a typical compression tester looks like..........



To do this test, you will need an assistant to help you. It is best to perform this test with the engine warm, but if it will not start, then you have no choice but to test it cold. First off, you need to remove the coil wire and then remove all the spark plugs. Be sure to mark your wires so you do not get confused. Find the appropriate adapter in your compression test kit to match the spark plug thread and insert it into the #1 cylinder. You just thread it right into the cylinder by hand until it's snug.

After you have the tester inserted, make sure your car is out of gear or in Park (Auto trans) and the parking brake is set. Have your assistant hold the gas pedal all the way to the floor (WOT - Wide Open Throttle) and begin cranking the engine over for a count of 5 to 7 seconds. Record your reading from the gauge and repeat this on all cylinders..............

Now add one teaspoon of engine oil into the cylinders and repeat the compression test one cylinder at a time. This will give you the "wet" readings and determine if you have worn rings.

The compression readings should be no less than 156 PSI on all cylinders and they should not vary much between the others. Here is an example of a low compression reading.........



A healthy reading would look like this............



A really healthy engine will have around 200 PSI of compression. Please note that these engines will run with lower compression numbers, but will not run correctly or get the expected fuel mileage.

[Woodie83]

You can't connect a reader and drive, that started in 96, but you can put a fuse in the raised diagnostic slot in the fusebox over your left foot then turn on the key and count the flashes of the "check engine light". It flashes out error codes in Morse Code. Could be a bad ground causing the computer to loose communications with a sensor.

Could also be worn out. As Johnny said, get a compression test.

[91-93-96Geoman]

Hello All. The check engine light finally went solid, so i was able to get the codes: #23 and #51. For code 23, I ohmed-out the intake air temp sensor and applied heat to it. The readings seem fine and the resistance decreased at the temp went up, so I reinstalled it. I checked power +5v to the temp sensor=good.
For #51, I checked the EGR for plugging and mechanical operation =good. I checked both intake ports for carbon blockage=none. I cleaned and checked operation of the vaccum modulator=good. Checked all vacuum hoses for blockage=none. I removed and applied +12v pwr to the little vacuum solonoid and found it did NOT close completely sometimes like there was debris preventing complete closure, so I replaced it.
I disconnected the battery and reset the computer. The check enging light has not come on, but still flickers once in a while. What is WEIRD is that it seems to flicker most when performing a RIGHT TURN under acceleration (with a full tank of gas). It does not do it on left turns.
I will be performing the compression test soon, just wanted to update the status here.
Thanks and regards...
Note: I will also be checking the fuel pump pressure and checking for a plugged fuel filter.

[91-93-96Geoman]

Update: 11-10-09.
Wife took the car for a run today and the engine light keeps coming on when the car seems to misfire. This misfire was occuring at 65MPH. The light would turn off immediately after the misfire and then ran fine/back to normal. Like I mention, this is an intermittent and occasional misfire, not a solid/repeated misfire. I guess I am back to the books on this one. Thanks.

[Woodie83]

They all have 51but it doesn't affect driveability. Usually nothing needs to be replaced, just clogged up with soot. Your flickering light still sounds like a bad ground to me.

[91-93-96Geoman]

Hello all and thank you very much for your replies.
The problem is fixed. It was the Air Intake Temperature Sensor.
Code 23 pointed to the air intake temperature sensor, which I OHM'ed out to be good. Evidently, I was incorrect. I replaced the sensor and the problem has been completely solved. This is the first time I have ever seen one of these go "BAD", but I guess they do.
This used $3.00 part from the wrecking yard fixed the engine surging at startup and the misfires at 65-70 mph on the highway.
Again, thanks for all the help.
Regards
91-93-96Geoman

[Colt1911]

interesting, I have been getting code 15, or 51 the jurys still out. I have replace all but the air charge intake sensor.thanks to your post, and similar beahvior, I may try that next. ($1 at the local pull yard :P) Why not?