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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: NCali on January 24, 2020, 04:56:12 pm

Title: open to closed loop on a tbi system
Post by: NCali on January 24, 2020, 04:56:12 pm

87 2wd half ton  completely stock 305 TBI

How can I tell if the system is going from open loop (when cold) to closed loop (after it heats up)
did a smog  check and came back with high hydro carbon and high co as you would expect to see when cold.

at 1601 rpm  co2 7.6  02 5.4   HC max 127 measured 219      co max .82  measured 7.21   no MAX748  measured 85

at 1076 rpm  co2 7.2   0210.8   hc max 98  measured 615       comax .72   measured .04    no Max761   measured 354

Title: Re: open to closed loop on a tbi system
Post by: VileZambonie on January 24, 2020, 06:41:23 pm

High HC and High O2 is a sign of a misfire... Check for a tune up and vacuum leaks. As far as how to tell if you are in closed loop, use a scan tool. If you don't have one, get one, you'll be glad you did.

Title: Re: open to closed loop on a tbi system
Post by: NCali on January 24, 2020, 08:26:53 pm

I did the cap rotor wires plugs before taking it in
cant find a vacuum leak using carb cleaner but I almost suspect the exhaust manifold on the passenger
side may be cracked . I see some discoloration by the tin.
would a cracked manifold cause this?

Title: Re: open to closed loop on a tbi system
Post by: VileZambonie on January 25, 2020, 06:05:01 am

High HC's is unburned fuel. High O2 means oxygen not consumed in combustion. CO is low CO2 is low indicating lean/lean misfire combustion problem. Does it run smooth?

A scan tool or WinALDL something you can display datastream will help you diagnose what is going on. You are missing out on pertinent information, fuel trims, input values -ECT, O2, MAP etc. You can find OBD1 GM 12 pin scan tools all over the place. I have several that have inches of dust on them myself so people are always unloading them cheap, look on marketplace or craigslist etc. Don't get just a code reader, you need one that has serial data display. There are many options for this.

Title: Re: open to closed loop on a tbi system
Post by: NCali on January 26, 2020, 04:28:04 pm

The truck does run smooth thats why I was thinking about a cracked manifold. I cant find any vacuum leaks

as far as fuel trims , I dont have that info.
I have a tech 2 scan tool for my other car bot no obd1 adapter.

I have been getting codes 44 that confirms a lean burn  code 32

well thanks for your help

anthony

Title: Re: open to closed loop on a tbi system
Post by: 87_Rado on May 20, 2020, 01:56:05 pm

Sorry this is a little old but I'm curious in the scan tools as well. Any recommendations on a decent prices obd1 scanner? Can you still purchase one new?

Title: Re: open to closed loop on a tbi system
Post by: VileZambonie on May 21, 2020, 10:58:14 am

There are MANY options still. See the post above You can also get the ALDL bluetooth dongle and ALDL app