TBI backfire and dies at idle

[PhotonFanatic]

I have the TBI 350 in my 87 R10 truck. It likes to die at idle, and makes a loud popping noise too, which I'm guessing is backfire. At first I thought it was bad gas because it sat for about 6 months without starting, but I put Stabil in it so that wouldn't happen. I also ran that tank out, and filled it up with 93 octane last week. I was thinking maybe the fuel filter was giving me grief, but I don't think the truck would run at all, if it was the fuel filter.

As it is, it's pretty cold blooded. It will stay idling with less popping, once it's warmed up. But it will still pop from time to time. A bit of foggy haze rises up out of the TBI unit, but I dunno if that's normal. The EGR valve has caused the check engine light to come on occassionally since I got the truck, but I'm not sure if that's related. The current tune up (plugs, wires, o2 sensors, etc) has about 15k miles on it. So I'd hesitate to say it needs a tune up. Sometimes there is a squealing noise, but I can't find anything wrong there either. Oil is full and there's good oil pressure.

The engine has well over 150k miles on it. Any help would be greatly appreciated!

 

[fast 99]

Put a vacuum gauge on it, is it steady? Sounds like a valve train issue of some sort flat cam, broken valve spring, loose guides, ect.

 

[squaredeal91]

have you checked or replaced your coolant temperature sensor? Very well could be that. They are Cheap and easy to replace on these chevys.

 

[Bextreme04]

Timing, Coolant temp sensor, Fuel pressure, IAC, EGR... could be any of those things. First thing I would do is properly check the timing and that the wires went back on in the correct firing order during the tuneup. Then I'd be checking the CTS and fuel pressure. All of that is free and just takes a small amount of time to test and verify.

 

[PhotonFanatic]

Timing, Coolant temp sensor, Fuel pressure, IAC, EGR... could be any of those things. First thing I would do is properly check the timing and that the wires went back on in the correct firing order during the tuneup. Then I'd be checking the CTS and fuel pressure. All of that is free and just takes a small amount of time to test and verify.

I know how to check timing and the plug wires, but how do you test the fuel pressure, IAC, EGR, and CTS for free? I'll certainly do all that.

 

[gmbellew]

I know how to check timing and the plug wires, but how do you test the fuel pressure, IAC, EGR, and CTS for free? I'll certainly do all that.

for fuel pressure you need a gauge. the rest is laid out in the drivability and emissions manual in the reference section and you will need a voltmeter. a more user friendly option than a voltmeter is ALDLDroid and a Bluetooth OBD1 reader, all in around a hundred bucks for the cable and app depending on how much inflation has raised prices in the last few years.

 

[Bextreme04]

I know how to check timing and the plug wires, but how do you test the fuel pressure, IAC, EGR, and CTS for free? I'll certainly do all that.

You can rent the fuel pressure tester from any major auto parts store. They will charge you ~$200 for it, then refund you the whole amount when you return it(its actually a tool rental, but they charge you for it just in case you don't return it). Everything else is a multimeter test based on the flow chart in the drivability manual like gmbellew said. The biggest issue I see with TBI is that no-one actually does any troubleshooting and they just throw parts at everything. Its also probably worthwhile to get a good contact cleaner/protector like DeoxIT and spray/clean every connector you take apart to test. After 30+ years of poorly sealed connectors in an engine bay, they are probably dirty or corroded as well.

 

[PhotonFanatic]

Thanks I downloaded all the manuals from a guy on another forum. Hopefully you're talking about the same manual. I'll just look for the flow chart in the drivability part of the manual. I have a timing light and the right distributor wrench so I can check those pretty easily. I have a digital multimeter so I'll check those sensors 2nd. Correct me if I'm wrong here, but it sounds like you're talking about checking a bunch of parts for the right electrical resistance. Too much = bad and replace. A bit like checking an ignition coil.

Then do the fuel pressure test last if those things don't pan out.

And if that don't work, take it to the mechanic and pay his diagnostic fee. And decide whether I want to fix it myself.

 

[gmbellew]

I think there is also a chart for "backfire" and other symptoms in the front of the drivability manual. it may have some common causes to check for that and other symptoms.

 

[PhotonFanatic]

Haven't checked the timing yet, but about 8 years ago I had the truck tuned up by a good mechanic and he set the timing. So I'm curious... what causes it to lose that setting? Why would the timing be off, without someone messing with the distributor?

And if it does turn out to be the timing, how often should it be checked and reset?

 

[Bextreme04]

Haven't checked the timing yet, but about 8 years ago I had the truck tuned up by a good mechanic and he set the timing. So I'm curious... what causes it to lose that setting? Why would the timing be off, without someone messing with the distributor?

And if it does turn out to be the timing, how often should it be checked and reset?

If its been 8 years since you have touched anything ignition related... that might be your problem. Wires go bad. Internal components go bad. The timing gets a base setting with the computer disconnected from the timing. The computer will then control the timing within a pretty wide window. Backfiring can happen from timing being way out, mechanical issue like broken valve springs, collapsed lifter, loose rockers, etc...

The manuals are all available on this forum in the reference library section.

 

[PhotonFanatic]

Well I got some more time to work on the truck. The timing light shows that the timing is off. Looking at it while the engine is running, it's about 1.25, maybe 1.5 inches away from the mark. To me that seems like a lot. So here's my question:

If I use the distributor to set the timing, what keeps it from happening again? Does this mean the timing chain is stretched and should be replaced?

 

[gmbellew]

Well I got some more time to work on the truck. The timing light shows that the timing is off. Looking at it while the engine is running, it's about 1.25, maybe 1.5 inches away from the mark. To me that seems like a lot. So here's my question:

If I use the distributor to set the timing, what keeps it from happening again? Does this mean the timing chain is stretched and should be replaced?

timing will be way off the tab of the EST wire is plugged in. unplug the EST and timing should be near 0

 

[squaredeal91]

It's a tan wire I think. Disconnect it to set base. Then reconnect to let ecm take over.

 

[gmbellew]

It's a tan wire I think. Disconnect it to set base. Then reconnect to let ecm take over.

yes, tan and black. a single connector coming out of the harness by the brake booster