GM TBI-era troubleshooting

Where should I start attempting to troubleshoot a no-start/running rough problem that seems to have corrected its self?

Wednesday the Suburban died a couple seconds after it started, then took about 20-seconds if cranking to restart, at which point it was running really rough. It was the first time I’d started it in about 10-days. I had places to go & didn’t want to risk getting stranded, so I shut it off & left it until today.

Of course it started fine this morning, so I took it for a drive around the block & it seemed fine. I pulled it in the garage to give it a look over, but nothing seemed amiss. 

I thought about checking fuel pressure, but I didn’t see a nipple on the fuel line to connect a gauge. I know ignition modules have a high failure rate, but that wouldn’t be intermittent, right? Do these have the problematic under-hood fuel pump relays like the C4 did?

Fuel pressure for a TBI is around 13psi if it’s not close it will run badly. That’s where I would start. There’s not test port. You have to T a gauge into the line by the fuel filter. 

In reply to sergio :

Thanks!

You can rent tbi pressure checker at orielleys. 

Also sheck the egr valve. Theyre bad about becoming sticky intermittently

I wouldn't dismiss the intermittent ignition module.

I got a free GMC Sonoma, due to a faulty ignition module. It just died while the PO was pulling out his work parking lot; he was "up to here" with the nickle and dime repairs." (Now ex-)step son calls me, asking if I wanted a free truck. I got there, got the keys, with no one else around, and it started just fine.

Long story, short, it ran great for a few weeks before it died and wouldn't start. Ignition module was the culprit. New module kept ir running for another year.

In reply to Dusterbd13 :

Oh, mines been throwing an EGR code since I got the truck last November. That’ll caus it to not run?

In reply to RealMiniNoMore :

Intriguing, good to know!

I had an intermittent pickup coil problem that resulted in a new distributor.  I chased it down by reading fuel pressure as it ran and had the problem with no pressure change, changed the module just because, and assumed it had to be the other thing inside the distributor.  I still have that distributor should you need it

Oh, mines been throwing an EGR code since I got the truck last November. That’ll caus it to not run?

If EGR is wide open at idle it'll run poorly enough that it may stall. If you have a hand vacuum pump you can open it yourself and if the truck doesn't run worse then it's probably already open.. I think you can still look through the side of the EGR valve in those and see the shaft moving up and down too. 

In reply to Patrick :

Thanks for the offer! At this point I just hope it acts up again soon so I can try to pinpoint the problem.

In reply to Vigo :

I do have a Mityvac, so I’ll give that a shot. Honestly though it always runs super smooth, and the CEL doesn’t come on until after I’ve been driving at highways speeds for 5+ minutes. Even then it still runs fine.

This occurrence on Wednesday was a complete anomaly.

I second the pickup issue, although mine would occasionally sputter like it was starving for fuel.  It finally left me stranded.  I tested the pickup coil as per the factory service manual (using a D cell battery) to confirm my diagnosis.  A new distributor fixed it for about $89.  With a new brass cap and rotor.  

I drove the truck today to pick up a battery for my step-son’s car, and the truck definitely has a miss. It seemed to develop after the truck was warm, but was only really noticeable under part-throttle deceleration, or a couple times cruising 35-40. When I got home it smelled very rich, even after I shut it off.

So I’m thinking plugs/wires/cap/rotor/ignition module?

Is there any way to make it misfire continuously? If so you can probably narrow it down to one cylinder using an infrared temp gun pointed at the exhaust manifolds where each port comes out of the head. 

If you have a multimeter you can just backprobe the o2 connector and see if it is actually running super rich. A look at the plugs would also tell that, plus if you pull ALL the plugs and one looks different than the others that may lead you to a problem area. 

A few years ago I spent many days chasing what seemed to be a fuel starving issue on a 97 C1500. Ultimately it turned out to be a low battery/low voltage issue that meant the fuel pump wasn't getting anywhere near 12volts. This would manifest itself in several ways. Either not enough total pressure to tell the ECU to start the engine, or it would start and run but then sputter at speed.

I only mention it as something to keep in mind - easy to check cranking voltage and charging voltage - something I did not do early on.

In reply to Vigo :

Thanks, I wouldn’t have thought to check the O2 sensor, though there’s no doubt as it’s running rich enough to water my eyes. I’ll definitely try my IR gun if I can get it miss at idle!

In reply to Sparkydog: 

That would have driven me crazy! The battery is 6-months old, but I’ll check the voltage just in case the alternator is failing. 

I tested the Egr valve this morning. It was completely dead & held zero vacuum, so I picked up a new one and replaced it. As soon as the truck warmed up, it died, flooded, and wouldn’t restart for several minutes. I finally got it to start by holding the throttle wide-open, and had to limp it home with one foot on the brake & one on the gas. This was as I pulled into the garage and it died.

So why would a new Egr valve make it worse???

-Sounds- pretty pig rich.

Look at the MAP sensor.  If you have a leak/crack in the lines to the MAP sensor, the ECU will think you're at a lot more throttle than you are and add a ton of fuel.  If the low vacuum doesn't make sense to the ECU (ie, doesn't match TPS and/or RPM expecteds), it'll throw an EGR code because the only thing that it can identify as causing low vacuum is a stuck EGR. TBI is pretty bulletproof, but dumb.

The fact that you were in there mucking with the EGR strengthens my theory, because MAP lines are in the same area.  You may have made the crack/leak worse.

If you've got a multimeter you can backprobe the MAP and make sure it's sending proper signal. In case you find the vacuum to it is actually fine. On the middle pin you should have ~0.7-1.0v at idle if the engine is running well enough to make proper intake vacuum. When the sensor doesn't see vacuum the signal voltage goes high (4-5v) so if your engine is idling and your sensor is putting out a high signal on the middle wire then it may be bad, or the ground wire may have high resistance or an open circuit. A busted ground on a MAP sensor makes it read 0"vac/WOT all the time. 

Thanks guys. I heading to pick up a T fitting in the morning so I can check fuel pressure on the Miata(and the Suburban if needed), then I’ll get back to troubleshooting...

I spent hours today on the truck, but it’s still not running. 

Blocking off the EGR and replacing the vacuum line to the MAP sensor is now allowing it to fire off ether, but it won’t stay running more than a second. I need to get a second set of eyes to watch for fuel from the injectors. 

Follow Vigo's tips and measure the MAP signal next.

I spent a little time on the truck today & confirmed I have fuel pressure at the TB, and the pump is running for ~2sec when the key is cycled on. I don’t have a noid light but I set my Dvm to a/c and I measure voltage across the injector plugs when cranking. Odd...

Here’s what I know:

• It ran ok until I changed the egr valve, then it ran like E36 M3. 
• After installing an egr blockoff plate(as well as after removing it & reinstalling the egr valve) it hasn’t run since. 
• It’ll fire on ether.
• Plugs are brand new after the egr valve was replaced. 
• I have fuel pressure at the TB. 
• The injectors are pulsing. 

While it certainly could be coincidental & unrelated to the egr problem, my gut says otherwise. 

However, I have fuel pressure and pulsing injectors, so short of a blockage in the injector I’m not sure what else to check?

Well, on a tbi system you can actually see the spray coming out of the injectors so, are they spraying? Normally, having fuel pressure and injector pulse would get you fuel and adding starting fluid wouldn't help. But, there is always the chance of an open winding in one of the injectors. If you can see fuel coming out of them then don't bother checking, but if you don't see fuel coming out of them check the resistance between their two terminals. A quick google check suggests they should be between 1-2ohms. If THAT checks out and you still dont have spray then check them with a stethoscope (real or improvised) and check for clicking during cranking. If they're clicking and still not spraying then pull them out of the throttle body and look at them. I think they have filter screens around the sides which could conceivably get plugged by debris.

Might also want to gently twist the distributor and see if it's loose enough that you could have accidentally nudged it way out of time while doing the EGR stuff. 

In reply to Vigo :

Ok, I’ll have to get someone to crank it while I peer inside the TB & see if they’re spraying. 

Thanks!

Do you have a flexible duct between the MAP and the throttle body?  Check it for cracks.  Had that happen on my old Camry.  Also, ignition module failures usually start as a temperature issue (module gets hot and won't work until the engine cools off).  Eventually, they'll fail completely but are normally replaced before they go terminal.