TBI HELP PLEASE

[jkhll8]

Ok I have an 87 305 TBI. Started running bad in January but couldn’t figure it out. Stalling, rough idle, missing at idle and all this was when it got warmed up. It ran fine while cold. Kept driving it until it finally quit wouldn’t start back. Replaced the distributor and all was good until yesterday and it started the same ****. Identical symptoms. Took the distributor back to autozone (yeah I know) got a replacement. Replaced it and it still has the same problem stalling at low speeds and at idle. Runs fine at speed. I’ve replaced IAC. EGR doesn’t seem to be sticking, changed plugs and checked compression. It just seems to be runnin extremely rich and a couple of the old plugs were fouled pretty bad. They were wet with fuel but every wire is firing with my timing light. It has a vibration off idle like an ignition miss. Any help is appreciated. One step closer to that LS swap lol


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[1987 GMC Jimmy]

CTS? I know I’ll get the scan tool hooked up to see what it’s saying on mine. It’ll dump fuel if it’s telling the ECM a cold temperature. I consider this a tune up component, and the only way I know to test them is to hook TunerPro up to the ECM.

What about the injectors? Are they spraying correctly, or are they stuck open? I guess the only real test for this is to watch them spray then unplug one at a time and start it back up to see if the one you unplugged stopped spraying. It should run really bad when you do this, and the injector you unplugged should be off. If it runs somewhat normal and drips or sprays fuel, that’s a problem. The fuel pressure regulator maybe at fault, too, so it might be worth rebuilding the TBI. It’s cheap and easy insurance against a clogged screen, ripped regulator diaphragm, bad or pinched o ring, etc. A fuel pressure test and a new filter never hurt, and while it's best to always start simple, I don't think this will cure your issue.

What about spark strength/quality? You should have a snappy white spark, and if you don’t, the coil might be at fault. What kind of distributor did you get? I’d only deal with ignition parts that say AC Delco, Delphi, or BWD on them. The rest of them can go to a landfill. Oh, and no codes? Computer turning on when you turn the key on as indicated by an SES light self-test? I'll attach the multimeter coil test for a TBI. I don't think it's a common phenomenon for a coil to go bad, but I've noticed multiple old HEI coils getting dead spots on them. They still work, and I keep them around as spares, but that doesn't sit well for me with keeping them.

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[chengny]

Ok I have an 87 305 TBI. Started running bad in January but couldn’t figure it out. Stalling, rough idle, missing at idle and all this was when it got warmed up. It ran fine while cold. Kept driving it until it finally quit wouldn’t start back. Replaced the distributor and all was good until yesterday and it started the same ****. Identical symptoms. Took the distributor back to autozone (yeah I know) got a replacement. Replaced it and it still has the same problem stalling at low speeds and at idle. Runs fine at speed. I’ve replaced IAC. EGR doesn’t seem to be sticking, changed plugs and checked compression. It just seems to be runnin extremely rich and a couple of the old plugs were fouled pretty bad. They were wet with fuel but every wire is firing with my timing light. It has a vibration off idle like an ignition miss. Any help is appreciated. One step closer to that LS swap lol


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It is running rough at low loads (i.e idle) and when up to operating temperature. You changed the IAC, and that's good place to start looking for the cause of that kind of behavior. But, I have to ask, did you do an autopsy on the old one - to determine if there was anything even wrong with it. Same question goes for the distributor. I never chuck any component until it has been definitely identified as NFG.

How about the TPS? The idle control system can't function properly without the TPS first telling the ECU that it's time for that lazy IAC ***** to wake up and do his job. Seriously, it could be a matter of the ECU getting inaccurate input from the TPS - the trouble might be electrical in nature or it might even mechanical binding. Output voltage should vary from a low of probably 1/2 a volt - when the throttle is closed - to 5-6 volts at WOT.

All 3 components need to work together to maintain design RPM at idle - you are out of the picture and can't help, because your foot is no longer controlling the volume of combustion air.

I spend a lot of time working with the old Saab Trionic systems - and I have become fairly familiar with how the sensors of that era interact with each other and the engine control module. And I learned years ago that the most critical sensor in most engine management systems is the MAP sensor. The ECU relies the input from the MAP sensor more than any other. Well, the O2 sensors can ultimately overrule everything but that's another story. The signal generated by the MAP is what the ECU primarily uses to determine both injection duration (quantity of fuel) and also ignition timing.

If you still have the distributor and IAC valve on the bench, you might consider taking a look at them and maybe cleaning them up. Then, when you have another mystery breakdown situation, pop them back in - but only one at a time. If engine performance improves, you can assume it wasn't whichever one you tried - and look elsewhere.

Another couple of components that are in the idle control loop are the P/N switch and the A/C control switch. How is your exhaust system breathing - a restricted exhaust will begin to cause trouble at higher temperatures.

Oh and a bit of advice, testing for a robust spark with an inductive timing light won't tell you much at all. It doesn't take a much juice to make that neon lamp light up. The coils in the clamp are designed to act like a transformer and really ramp up the voltage. Next time you want to confirm a healthy spark, pull a plug and watch the arc as it jumps across the electrodes. The arc generated by 40,000 volts jumping across that gap should be a fat bright blue - bright and fat enough to easily be observed even in broad daylight. You shouldn't even have to watch it, it should create an audible snap sound each time it fires.

Just a couple of things to consider.

 

[1987 GMC Jimmy]

As a brief addendum to what Jerry said, all the literature I've seen suggests a resting TPS voltage of close to or at .5V, but it seems in my case and others, at least some aftermarket sensors rest at .6V or maybe even a hair more. I tried boring out the mounting holes on mine to adjust it and get the voltage as close to .5 as possible, but the best I could do was .59V.

 

[4WDKC]

get a scanner or laptop connected so you can see what the computer is saying. You say you replaced the IAC did you do the relearn/calibrate process?

 

[jkhll8]

CTS? I know I’ll get the scan tool hooked up to see what it’s saying on mine. It’ll dump fuel if it’s telling the ECM a cold temperature. I consider this a tune up component, and the only way I know to test them is to hook TunerPro up to the ECM.

What about the injectors? Are they spraying correctly, or are they stuck open? I guess the only real test for this is to watch them spray then unplug one at a time and start it back up to see if the one you unplugged stopped spraying. It should run really bad when you do this, and the injector you unplugged should be off. If it runs somewhat normal and drips or sprays fuel, that’s a problem. The fuel pressure regulator maybe at fault, too, so it might be worth rebuilding the TBI. It’s cheap and easy insurance against a clogged screen, ripped regulator diaphragm, bad or pinched o ring, etc. A fuel pressure test and a new filter never hurt, and while it's best to always start simple, I don't think this will cure your issue.

What about spark strength/quality? You should have a snappy white spark, and if you don’t, the coil might be at fault. What kind of distributor did you get? I’d only deal with ignition parts that say AC Delco, Delphi, or BWD on them. The rest of them can go to a landfill. Oh, and no codes? Computer turning on when you turn the key on as indicated by an SES light self-test? I'll attach the multimeter coil test for a TBI. I don't think it's a common phenomenon for a coil to go bad, but I've noticed multiple old HEI coils getting dead spots on them. They still work, and I keep them around as spares, but that doesn't sit well for me with keeping them.

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I pulled codes with a paper clip and only got a 12. I haven’t got a scan tool to check any thing. I changed the fuel regulator last year. The injectors seem to be working fine and the coil hasn’t been changed looks to be the original one. May go that route next.


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[jkhll8]

get a scanner or laptop connected so you can see what the computer is saying. You say you replaced the IAC did you do the relearn/calibrate process?

Yes I done the relearn


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