Lean Misfire

[Markb873]

Hello,

I have a '93 S-10 Blazer. I have a lean misfire under light throttle right before a shift. I also have a random misfire at cruise which wasn't there when it was warmer outside. Also when going up a fairly steep hill I get a surge that runs in about a 5 second cycle. It's noticable but not bad.

I have replaced:
ignition system
intake gaskets and and replaced a leaky head gasket.
EGR
Fuel Pressure regulator.

Vacuum is good.
Compression is good.
Fuel pressure is good.
No Fuel wash in plenum.
Sea Foam makes it worse.
Premium makes it a bit better.

Anybody have any ideas on this one??

[viggy58]

fuel filter?

that a v6 or 4 cyl?

[danielsatur]

A 1993 Blazer has a OBD I system, it won't recognize a DTC P0171 (Lean code) like a OBD II system. >1995

Do a smoke test for unmetered air leaks.

[old_master]

Here are some of the basics on your 4.3L "W" engine:
Have you checked fuel pressure and leakdown? What were the results?
Did you use AC Delco spark plugs CR43TS with a gap of .045?
Have you replaced the spark plug wires?
What is the operating temperature of the engine?
How many miles on the oxygen sensors?
Have you checked the sensor/computer grounds at the thermostat housing?
Have you checked the sensor/computer grounds at the rear of the cylinder heads?
Is the check engine light on? If so, have you checked DTC's?

[Markb873]

I rechecked the fuel pressure. Was lower this time at about 52 psi. There was a bounce when a particular poppet would operate. It was about a 1/2 psi higher. Also moving the throttle increased the pressure about 5 psi and it would hold. When I would quick flick the throttle the engine would lope badly and almost stall which I never noticed before. As far as leakdown, It lost 5 psi over 10 minutes.

Spark plugs are AC Delco and gapped properly

Spark plug wires have less then 1000 miles on them.

Guage on dash doesn't work for temp but I put in a new thermostat this summer. It didn't run hot when it worked and I never hear the coolant boiling.

Grounds are all good....clean and tight.

Check engine light is off.

I put in a new air charge sensor and it improved it about 50%

O2 sensor is dated

-Mark

[Markb873]

I checked the fuel pressure again because I still had it hooked up. I had the guage oriented a different way and now the fuel pressure was higher at 58 psi with the pump running....bad guage too.

[Markb873]

I fixed the temp guage and it indicates a bit low...roughly 190...Hard to tell with three numbers listed on it. Where is the connector for the O2 sensor??? Looks like it is on top of the transmission??

-Mark

[Markb873]

Alright, ended up replacing the IAT Sensor and got rid of most of the misfire. It will stumble a fire or two but only does it about 10 times in 20 miles. I am now getting a sudden, mild burst of acceleration when cold the first time I hit the brake and it tries to idle.

The idle quality is generally poor but will occasionally run perfect. I am now thinking a stubborn IAC valve??

Still have no codes though.

-Mark

[olopezm]

I believe the IAC will only throw codes once it has completely stopped working (either bad motor or stuck) but as soon as it can still move it won't cause any codes, so just go ahead and give it a good clean.

Best regards,

Oscar.