I have a slight misfire when engine is started cold ?

[Killain]

I have a 350 gas V8 and in the mornings when I start the engine, I have a slight miss until I put it in gear, then the miss-fire goes away, and the engine runs fine ? I restored the truck over the last few years and I put in a matched set of TBI injectors and an new Edelbrock 3407 TBI intake manifold. What should I be looking for ? I run it on 89 octane fuel and have run 3 bottles of Chevron Techton and Royal purple fuel system cleaner through it and still I have the miss.

 

[Will L.]

When you put it in gear, the rpm will lower slightly. If raise the rpm to the same it was before, does it have the misfire? Some rough idle is just more noticable at certian rpm.
Say 900 rpm idle in park, shift in gear with foot on brake it goes to 775 rpm. While holding brake, raise rpm to 900 and listen for misfire or rough idle. Dont do this too long, loading the torque converter will start to heat tranny fluid.

 

[Killain]

When you put it in gear, the rpm will lower slightly. If raise the rpm to the same it was before, does it have the misfire? Some rough idle is just more noticable at certian rpm.
Say 900 rpm idle in park, shift in gear with foot on brake it goes to 775 rpm. While holding brake, raise rpm to 900 and listen for misfire or rough idle. Dont do this too long, loading the torque converter will start to heat tranny fluid.

Hay thanks for getting back, No, it just misfires in cold and once in gear there is no sign of any misfiring. I put a MSD coil on it, should I go with their cap and rotor ? OEM parts come from, hell I don't even know anymore. Thank you !

 

[Will L.]

Ok, you said misfires in "cold"

Are you letting the engine coolant temperature warm all the way up before putting it in gear, and when it warms up to normal operating temperature, then it runs proper?

 

[Killain]

Yes, it only misfires for the first 2 minutes in idle, I don't idle it for more than a minute or so. It's TBI injected so it's not doing any good to idle it any longer than 45 seconds or so. And once I put it in gear the miss goes away, but it will come back until it's fully warmed up ?

 

[Will L.]

As for only needing to warm 45 seconds because tbi, you need to do a couple hours research online about these old gm tbi and the term "choke mode". The knowledge you gain on that will help you in years to come. That is part of what you are experiencing now. I almost never warm up any vehicle to operating temp before driving them- tbi was the only exception in gm gas engines.

Because the ect is not 160 yet, but no stumbling when you drive it, I dont suspect ignition system.

Also no complaint on top freeway speed, so i dont think fuel supply which ususally shows up in these 2 places first,high pressure pm loaded and cold idle. A simple test to be sure is with air cleaner off, on passanger side of throttle body is the fuel return line. If you remove it with engine running it should be pumping out fuel (so obviously have something ready to catch the fuel). If no fuel is coming out there a filter is plugged or bad pump, somethingthing along those lines.

You could have a small vacuum leak, bad signal from temp sensor (ohm to temp scale test), bad connection on grounding wires on your thermostat housing or near it iirc. The egr could be inop at idle.

I know there is something Im forgetting to check it here, sorry been too many years since playing with one. It should hit me in a bit (I hope- friggin old-timers kickin in haha)

 

[Killain]

As for only needing to warm 45 seconds because tbi, you need to do a couple hours research online about these old gm tbi and the term "choke mode". The knowledge you gain on that will help you in years to come. That is part of what you are experiencing now. I almost never warm up any vehicle to operating temp before driving them- tbi was the only exception in gm gas engines.

Because the ect is not 160 yet, but no stumbling when you drive it, I dont suspect ignition system.

Also no complaint on top freeway speed, so i dont think fuel supply which ususally shows up in these 2 places first,high pressure pm loaded and cold idle. A simple test to be sure is with air cleaner off, on passanger side of throttle body is the fuel return line. If you remove it with engine running it should be pumping out fuel (so obviously have something ready to catch the fuel). If no fuel is coming out there a filter is plugged or bad pump, somethingthing along those lines.

You could have a small vacuum leak, bad signal from temp sensor (ohm to temp scale test), bad connection on grounding wires on your thermostat housing or near it iirc. The egr could be inop at idle.

I know there is something Im forgetting to check it here, sorry been too many years since playing with one. It should hit me in a bit (I hope- friggin old-timers kickin in haha)

I hear ya, I'm getting ready to turn 65, and thank you, I think I could have a vacuum leak. I'll get the propane torch out and see if I can find it. I was under the unstanding that all fuel injected engine did away with waiting for the choke to open ? Thank you !

 

[Will L.]

TBI does not have a choke like a carburetor. The temp sensor tells it to add more fuel at colder temperature, hence ohm scale temp sender test. This method of replacing the choke is called choke mode. There are multiple issue that arise in the control of it, that cause a variety of problems. Becoming very knowledgable on it will help you diagnose many issues.

I need somebody elses help here on this for him. I just cant remeber the details.

 

[WarWagon]

New parts don't mean "good parts"! Case in point how to find bad new TBI injectors is here: http://www.thetruckstop.us/forum/threads/1991-r-v-burb-350-runs-better-with-alt-disconnected.45472/

First and foremost I bet your system is working 100% as designed. The New Leaner Oxygenated fuel is crossing the lean misfire line that OEM's had to be so close to to pass emissions for that model year. The EPA doesn't give a shit that your old vehicle won't run on today's fuel or low zinc engine oil.

Modern oxygenated fuel is the exact reason OEM's had to go heated O2 sensors and port fuel injection. Even with choke mode it's still too damn lean, on modern fuel, for the huge heat sink of the cold intake manifold - too much fuel is condensing back out plus the "leaner" gasoline is causing misfire. TBI in effect is just an electronic carburetor as the cheapest injection system GM could get away with even though better technology existed on other production vehicles. GM couldn't make the SUV's and pickups fast enough at some times and had no competition at all for Suburbans for a time. So GM got away with it. Don't even get me started on the Sub-Par, quality lacking, squeak, and rattle S-10/15 line. The S-10 blazers in some of the 4.3 V6's had a better CPI port injection system - I didn't say reliable. In context of this issue: better when working.

Couple cures for you:
1) Wire in a heated O2 sensor. Closed loop will come on faster and richen up the EPA mandated "lean oxygenated fuel". Find an hot with ignition wire like the 4x4 motor conversion would use (even if you are 4x2), relay, and heat the sensor. This eliminates the lean stumble even on 454's. Flip side is the O2 sensor cooling off can go back into open loop and be too rich. During warm up at idle you can be both rich or lean depending on the temperature, fuel, and part of the program running. To eliminate the air pump the ECM quickly goes rich, lean, rich, lean all the time so the converter can store O2 to burn the rich cycle needed to actually run.
2) Put in an adjustable fuel pressure regulator with a gauge and crank the pressure up. Richen up the mixture that way.
3) Starting the engine - long crank till it fires. Crank it a moment and STOP. Wait and then crank it again - this will have fuel in the manifold and help the lean start condition to fire it up. Otherwise you are having oil pressure come up off the starter - some say a good thing.
4) Look into a tune and see if the tuner or company offering canned tunes address this lean O2 fuel and starting/warm up/choke area.
5)Lean conditions cause the HV energy on the secondary side to find somewhere else to go aside of the spark plug gap. You spent the money on the coil now finish the job with the cap, rotor, and ceramic boot 8+ MM spark plug wires and route them as far away from each other as possible: this means buying lots of spark plug stand offs and wire clips. I will be blunt: Spend the money on a good set of expensive Accel/MSD spark plug wires or DON'T BOTHER! The cheap autostore stuff is worse then your old OEM. Spark plugs are another area: stay away from the catalyst Platinum. You have 30K life plugs and a good performance plug along that service interval is the way to go. AC Spark Plug is no more: Skip the middle man and go direct to NGK who makes ACDelco spark plugs now. GM isn't known for weak ignition systems, but, here is an example of a Dodge with a weak ignition. Again Lean Conditions cause the HV to fire elsewhere of the plug gap if it can. http://www.thetruckstop.us/forum/threads/2000-3-9-dodge-v6-misfire.44482/
6) When was the fuel pump replaced last? You have checked the fuel pressure? You are pushing your luck with a factory fuel pump on anything over 50K miles. Generally they just die and take the fuel tank connector with them. Some may make excessive noise pre-failure.

There could be a debate on 85/87 vs. 89 vs. 91/92 octane fuel IF one grade is leaner than the other. The garbage grade 85 is an oil company excuse at high altitude to screw everyone esp. if you are turbo/supercharged. An aftermarket exhaust that flows better messes with EGR backpressure and causes ping at high sustained loads in hot weather. This could be a justification for higher octane fuel.