97 Bravada intermittent misfire

[JLKINSER1970]

Hey all
The old Bravada strikes again. I have been having a misfire issue but only intermittent. I will be driving it and all is fine. May turn it off and go back to start it a few hours later and then it is missing all over the place. Rev it up and it straightens out until I get off the gas and then it misses again. So it only misses at idle. So I put my Genesis scanner on it and got tbs sensor and PO300, multiple misfires. Run the scanner in live mode and check my cylinder fire and all of the cylinders are firing except 5 and 6. So I put a test light in between the plug wire and spark plug on number 5 and 6 cylinder and all is firing fine. Pulled the plugs and no carbon build up. Pulled the distributor cap and it looks good as does the rotor. (The Genesis said this is the number one reported fix.) The coil was loose in it's housing so I replaced it. Put it all back together and still misses. Leave for a few hours and come back and she is running fine again. This goes off and on. Any ideas? Some one told me tonight it has to be the distributor cap and rotor. My thinking, although I hope I am wrong, wouldn't all cylinders misfire and wouldn't it be constant instead of intermittent misfire??? HELP!!! And thank you!!

[Chris Stewart]

I'm suspecting a vacuum leak or stuck open pcv valve. Will have a better idea when my daughter gets home in her 2001 S10.

[Chris Stewart]

Ok, just looked at the S10 and the power brake booster gets it's vacuum from the rear of the plenum. The pcv valve appears to get it's vacuum at the plenum very near #5 runner entry. I'd unplug the vacuum hose from the brake booster, put my thumb over the hose and have an assistant start the warm engine.
If the miss is still there, reconnect the hose, purchase and install a new pcv valve and retest.

[JLKINSER1970]

Hey Chris!!
Thanks for the reply! Well, I installed a new PCV valve a few days ago. I forgot to mention that in my previous post. Tonight I replaced the distributor cap, rotor, plug 5 and 6, and even the spark plug wires. The miss is still there . The only other thing I can think of is the fuel injectors on number 5 and 6 have went bad. I hate to even think that because of the price of the spider injector assembly. I saw Rock Auto had rebuilt ones. Any suggestions my friend?

[JLKINSER1970]

Oh one thing I forgot to mention. When the engine is running, I pulled out the PCV valve and nothing. The engine didn't bog down or attempt to die. I would think that me pulling that would have caused a major vacuum leak and the engine would have done something. Am I wrong?

[Chris Stewart]

Pulling the pcv valve out of the valve cover does nothing, it's still operating while it's connected to the vacuum line. Remove the vacuum line and the engine will miss big time.
Did you check the power brake booster line as described in the previous post?

A thought just occured, the MAP sensor should see the loss of vacuum and O2 sensor see a lean condition when a hose is pulled and the computer adjust the fuel squirt accordingly but I may be wrong. I'd do the vacuum line check anyway.

[ericn1300]

"Put it all back together and still misses. Leave for a few hours and come back and she is running fine again. This goes off and on. Any ideas? Some one told me tonight it has to be the distributor cap and rotor"

The distributor cap is a known problem on these vehicles especially in high humidity areas, or after a big rainfall. It's a cheap part and if it doesn't fix the problem you can always return it. Stick with ACDelco for ignition parts, it does make a difference.

[JLKINSER1970]

Okay guys. Sorry for the long delay. I have been working late the last several weeks.
I used my scan tool and did find out that my fuel injectors 5 and 6 were shot. Last night I replaced the fuel injector and now the engine will barley run. It will rev up (idle around 3000 rpm) and then goes down. It keeps doing this. I put my scan tool back on and all injectors are firing. No misfires. But I am getting the PO108 and PO107 codes. I have checked and rechecked the vacuum lines and no leak. It does sound like I am getting an air leak around the top the the intake where the injectors electrical housing comes through the intake. Also, like an idiot, when I replaced the upper intake I didn't replace the gasket because the old one looked good. I rebuilt this engine about 50,000 miles ago. Compression is good. Didn't have this issue until I changed the injector. What would you suggest? Should I take the upper intake off again and put on a new gasket? I was told that if the gasket was bad I wouldn't get a PO107 and 108 code but that doesn't sound right to me. Thanks for your input!

[Chris Stewart]

That's your MAP sensor, back inna few minutes.

[JLKINSER1970]

Could I have done something to the MAP sensor? I didn't even take it off when I took off the upper intake. Ally I did to the MAP sensor was take the electrical connector off when I was removing the intake and then plugged it back in when I put the intake back on. Could I have an intake leak? I ran her today and sprayed carb cleaner around the intake and around the fuel line inlet and didn't get a response.

[Chris Stewart]

The MAP sensor may be bad.

[JLKINSER1970]

I checked the electrical connector and it has power. Moved the wires around and the light never went out so it isn't the circuit. I am going to test the MAP sensor tomorrow, hopefully. Will let you know what I find out. Thanks for the responses!!