idle misfire, scope says its random

My 1996 Jimmy is awful in traffic. It has new Delco wires, Champion platnum plugs, Mobil 1, distrubutor cap and rotor, recent pro fuel injection cleaning, and lots of opinions as to what is going on (fuel pump, sensors, injectors, air leak, etc.). It is flooding, everyone agrees and it has cooked the converter. Cold starts are immediate, hot starts tough. Highway performance is fine, fuel economy primarily highway averages 18 mpg.

The most recent visit to a shop came up with I think it has to be in the intake, but the random misfire indicates something other than the "spider".

Could I get some experienced advice?

You don't have a spider, they were killed in 1995.

Have your fuel pressure checked with the key on and the engine off.

It should be between 61-66psi.

My 1996 Jimmy is awful in traffic. It has new Delco wires, Champion platnum plugs, Mobil 1, distrubutor cap and rotor, recent pro fuel injection cleaning, and lots of opinions as to what is going on (fuel pump, sensors, injectors, air leak, etc.). It is flooding, everyone agrees and it has cooked the converter. Cold starts are immediate, hot starts tough. Highway performance is fine, fuel economy primarily highway averages 18 mpg.

The most recent visit to a shop came up with I think it has to be in the intake, but the random misfire indicates something other than the "spider".

Could I get some experienced advice?


I will have the fuel pressure checked today. If it is unacceptably low, is the fuel pump normally the culprit (after fuel filter)?

Thanks for the advice. Judging by this site, fuel pressure is the most common problem on these vehicles.
Sounds like the fuel trickles instead of spraying at idle due to low pressure due to possibly low voltage, obstruction or failing fuel pump, right? New fuel pump has been recommended by 2 shops, but I thought that either it worked or it did not work...

Did you find out the pressure yet?

Did you find out the pressure yet?
I spoke with the mechanic who scoped it - but he did not test it, so I need to know where to connect the pressure guage to check it myself.

The garage guy still says it has a spyder, which makes me a little leary based on what you said. The forum seems to indicate there are a lot of wiring problems repaired at garages by replacing expensive pumps and things, so I want to get in on fixing it right, as economically as feasible.

a lot of jimmy's seem to get moisture in the distributor cap. take it off and blow it out with compressed air. if that doesn't fix it try removing the egr valve and cleaning it, they seem to get gummed up and stick a little bit. i don't know what shops you have been going to but a gm dealer has access to lots of information and a lot more experience with your vehicle than independent shops

I spoke with the mechanic who scoped it - but he did not test it, so I need to know where to connect the pressure guage to check it myself.

The garage guy still says it has a spyder, which makes me a little leary based on what you said. The forum seems to indicate there are a lot of wiring problems repaired at garages by replacing expensive pumps and things, so I want to get in on fixing it right, as economically as feasible.

The fuel pressure schrader connection is at the back driver's side of the engine where the fuel lines enter the upper plenum.

You will have to remove a cap like on your tire valve stem to get access to the screw fitting.

The fuel pressure schrader connection is at the back driver's side of the engine where the fuel lines enter the upper plenum.

You will have to remove a cap like on your tire valve stem to get access to the screw fitting.

I have located the screw fitting. My vaccuum/pressure gauge does not have any fittings. Is the adapter a readily available item at CarQuest or similar places? Or do I need a special gauge to test?

a lot of jimmy's seem to get moisture in the distributor cap. take it off and blow it out with compressed air. if that doesn't fix it try removing the egr valve and cleaning it, they seem to get gummed up and stick a little bit. i don't know what shops you have been going to but a gm dealer has access to lots of information and a lot more experience with your vehicle than independent shops

I had the cap and rotor replaced 2 weeks ago by a shop, and the points on the old cap were pretty bad. Now the truck starts easier but still idles rough like 1 or 2 cylinders are not firing regularly at idle, but joining in as rpms pick up to 1500 or so. The EGR - located on top of the intake? My egr past experience was removal and seal the hole (before OBD2) on inline engines! I can try cleaning the egr and testing fuel pressure to at least get direction on what needs fixing.

I was also considering hotter heat copper plugs in place of the champion platinums that have 5000 miles on them.

1.) Have you ever changed the fuel filter?

2.) Get the fuel pressure checked with a fuel pressure gauge.

3.) Suggest going to AC Delco Platinum plugs gapped properly.

DO NOT START INSTALLING HOTTER PLUGS.

1.) Have you ever changed the fuel filter?

2.) Get the fuel pressure checked with a fuel pressure gauge.

3.) Suggest going to AC Delco Platinum plugs gapped properly.

DO NOT START INSTALLING HOTTER PLUGS.

Fuel filter was changed 8000 miles ago. I still have the old one which does not seem to be plugged when air is blown through it.
I will buy some Delco plugs to replace the Champions.

did you check the fuel pressure yet.

Cracked distributor cap OR cracked COIL. Check for arcing. It took me quite a while to find the cracked coil as the problem was intermittent, only happening when damp outside.

Cracked distributor cap OR cracked COIL. Check for arcing. It took me quite a while to find the cracked coil as the problem was intermittent, only happening when damp outside.

I have replaced the cap and rotor. Starting is much better. How did you discover the cracked coil?

If fire is good, the engine should not miss. If coil is good, everything else is new AC DELCO, so either the computer is confused or it is a fuel injection issue, right?

I have been trying in vain to have a garage test fuel pressure. I am just going to have to buy my own dadgum gauge and do it myself. If it is fuel - either too much or not properly sprayed, that might result in idle misfire...

Check the wire between the coil and the cap.

The connections might be corroded on the coil.

I know mine was.

I have replaced the cap and rotor. Starting is much better. How did you discover the cracked coil?

If fire is good, the engine should not miss. If coil is good, everything else is new AC DELCO, so either the computer is confused or it is a fuel injection issue, right?

I have been trying in vain to have a garage test fuel pressure. I am just going to have to buy my own dadgum gauge and do it myself. If it is fuel - either too much or not properly sprayed, that might result in idle misfire...

I've had the same problem AGAIN after changing the Dist. Cap twice AND the Coil. The local mechanic replaced the Cap AGAIN for free without an explanation. I then talked to the original mechanic, ( who had refered me to the 2nd one ) and he informed me that NAPA told him that the OEM Dist. Caps are VENTED and the aftermarket ones have been being produced UNVENTED and EVERYBODY has been having problems !!! ( See post above about cap moisture. ) The problem has apparently been rectified, at NAPA anyway. Make sure you ask for and use a VENTED one.

have you been replacing the rotor with the cap?

I have replaced the cap and rotor. Starting is much better. How did you discover the cracked coil?

If fire is good, the engine should not miss. If coil is good, everything else is new AC DELCO, so either the computer is confused or it is a fuel injection issue, right?

I have been trying in vain to have a garage test fuel pressure. I am just going to have to buy my own dadgum gauge and do it myself. If it is fuel - either too much or not properly sprayed, that might result in idle misfire...


Final answer on the engine turned out to be Failed Fuel Pressure Regulator putting 62 psi at the injectors flooding the engine. New catalyst(watch out that distributor cap is not injured when replacing it - mine was broken and dealer insists muffler shop broke it) and pressure regulator (injectors were blasted clean by excess fuel) gives much better starting and gas mileage.

Thanks for the help.

Final answer on the engine turned out to be Failed Fuel Pressure Regulator putting 62 psi at the injectors flooding the engine. New catalyst(watch out that distributor cap is not injured when replacing it - mine was broken and dealer insists muffler shop broke it) and pressure regulator (injectors were blasted clean by excess fuel) gives much better starting and gas mileage.

Thanks for the help.

Change the oil ASAP.

It will be loaded with gas and damaging your bearings.

Change the oil ASAP.

It will be loaded with gas and damaging your bearings.

Blazer,

I have always used synthetic oil just in case. I recently noticed oil smelling like gas so I changed it and filter before going to have it tested.
Now that it is fixed, the engine is silky smooth. I guess the engine flushed itself out. Hopefully the bearings are ok - I have not heard any ominous knockings.

Do you have any ideas on the ac switching from defrost to dashboard vents? I get cold feet and frosted windshield but hot cabin!

Thanks for the help with the engine. I think I will polish the truck this weekend to celebrate having it run right again!

Blazer,

I have always used synthetic oil just in case. I recently noticed oil smelling like gas so I changed it and filter before going to have it tested.
Now that it is fixed, the engine is silky smooth. I guess the engine flushed itself out. Hopefully the bearings are ok - I have not heard any ominous knockings.

Do you have any ideas on the ac switching from defrost to dashboard vents? I get cold feet and frosted windshield but hot cabin!

Thanks for the help with the engine. I think I will polish the truck this weekend to celebrate having it run right again!

Yea, the gas in the oil is not good, just make sure you don't use a FRAM oil filter.

As for the vents, you have a vacuum leak.

Check all vacuum hoses in the engine compartment for leaks or cracks or from them not being attached.