87 tbi 350, engine stalls

[bbchevy]

I've replaced the fuel filter, cap, rotor, plugs, wires, coil, and changed the throttle body to one off another running truck. And i havent found any vacuum leaks. but still the truck insists on running perfect for 5 seconds then stalling out. if you give it a little gas it will stay on longer, but has no throttle response at all. bogs right out when you try to rev it. and it just gets worse when it warms up a little bit (if you can even keep it on that long). I have no idea about what the problem is.

[old Rusty C10]

well id check the pressure at the rails and if that is consitently good then id say clogged injectors

[bbchevy]

Like i said, the throttle body i switched it to, was off a running truck. so i know theres no problem there. and there arent any fuel rails on a tbi motor. . but i dont have a gauge anyway to check it at the fuel lines. but the fuel doesnt drip out of the injectors, it sprays like its supposed to.

[85chevy214]

Hey bbchevy have you ever replaced oxygen sensor, if not that then see about checking that and other sensors, but you should get a check engine light if there is sensor probs, I'am new to this site but your problems sounded familiar to me when i had a tpi iroc, so I thought I would throw my 2 pennies out there. Hope those suggestions help out.

[bmxkingnow]

I dont have too much experience with the TBI yet, but it sounds like a similar problem I had in my toyota supra. im not trying to relate toyota to chevy, but when my mass air flow sensor was unplugged, it would start and run for a few seconds and die. not only is it not getting the correct air flow reading, it was obscuring the o2 sensor as well. so I would have to agree the o2 sensor is bad. im not sure how or if the TBI reads incoming air, but that would be my first concern. someway the computer is saying you are getting too much fuel. so in that point, it could be a bad computer read out.

I havent played with a TBI yet, but intend on swapping a 305 with a TBI set up. I am curious to know what you come up with for a fix.

[D13]

When you turn on the key the fuel pump runs 2 sec to prime, fed by the key on and a timer. Once it starts it gets power via the oil pressure switch. Get a gage and watch the pressure.
Once the truck hits 400rpm the ignition timing swaps from the distributor module to the ECM. If the module does not do what the ECM tells it, does not agree to release timing, or worse, tries to fire both on its own and when the ECM tells it, bad things happen. Bad plugs or wires will backfeed and pop the module. It's fairly cheap, but use a Delco one 'cause auto zone specials do not have as good internal filtering.

[James McClure]

D13 probably hit the nail on the head. Sounds like a fuel starvation issue. Be warned though that there are 2 oil pressure senders, 1 for the pressure guage\light and one to feed B+ to the fuel pump. If you have a guage look down by the oil filter and you will see a rather large sender with a single wire, thats for the guage. I believe wheather you have a light or guage this wire is brown, but don't quote me. The sender you want is most likely under the distributor and is pretty small and has 2 wires attached to it. Wires are probably red & orange, red & red or orange & orange and in the 10 to 12 gage size. Check also fuel pump relay (ECM fed) and ALL the fuses. A quick test for the fuel pump would be to tap a test light into the fuel pump circut at the tank and see if the lamp stays lit as it goes through it's start-stall out cycle. Do yourself a favor, DO NOT,DO NOT penetrate wires insulation to check this, you'll be sorry 6 months down the road. jim

[85chevy214]

I think bmx is on the right track with the O2 sensor and maf sensors, I believe the O2 sensor triggered the ECM when the engine exhaust reached a predetermined temp. then ECM took over control. From what I learned emissions
and fuel mileage are the main reason for ECM vehicles. Exhaust temp. is very critical to emissions and driveability.

[ChevyTech]

1987 TBI trucks have no MAF sensor.

If it dies in five seconds, it never gets to “closed loop” operation, so the O2 sensor is ignored.

You swapped the TBI so it’s not an injector problem.

Originally Posted by D13

When you turn on the key the fuel pump runs 2 sec to prime, fed by the key on and a timer. Once it starts it gets power via the oil pressure switch. Get a gage and watch the pressure.
Once the truck hits 400rpm the ignition timing swaps from the distributor module to the ECM. If the module does not do what the ECM tells it, does not agree to release timing, or worse, tries to fire both on its own and when the ECM tells it, bad things happen. Bad plugs or wires will backfeed and pop the module. It's fairly cheap, but use a Delco one 'cause auto zone specials do not have as good internal filtering.

The distributor module could be the problem. D13 is correct that the module switches modes when the engine reaches 400 RPM.

See if it will keeps running if you put starting fluid or fuel down the TBI. If it keeps running there is a fuel problem. If it still dies get the distributor module tested.

Another thought that comes to mind is high current draw causing the ECM to shut down to protect itself.

Have you checked to see if the ECM has stored any trouble codes?

[85chevy214]

sorry for any mis info, every day I learn something new and I am grateful thanx

[ChevyTech]

Don’t be sorry for trying to help.

If everybody gave there best guess, someone might even be right.

[bmxkingnow]

sounds like d13 and chevytech both know what they are talking about, i am only guessing. any info helps right?

I did a little more reading on TBI my self, am i correct in understanding that the butter fly valves (much like a carb) would act like an air flow sensor. not in the fact that they meter airflow, but more as in the throttle position sensor. more air= more fuel? in this case that would eliminate any mafs, in which i could understand. which is also how you guys came up with it being a fuel delivery problem. so out of the two fuel pump sensors, if it cranks over and starts for a few rotations, that would eliminate the acc. sensor and pin the problem on either the oil pressure sensor or the ecu? or possibly a bad connection between the two.

also i read that any modifications done (ie. cold air or headers) wouldnt be imeadiatly rectified with the TBI because it is a learning system. once the ecu has a chance to process new info over a period of time, it would slowly make changes for the mods. thats one of the reasons they make after market chips for them? so in this sense, if anything was done to bbchevy's motor (such as getting rid of smog equipment or any sensors during the change) the ecu would read that as a dead sensor? would that send an ec code?

just curious for a future swap.

[D13]

Throttle blades regulate airflow. ECM knows "where" the throttle blades are via TPS (Throttle position sensor). Air flow is measured by Mass Air Flow (MAF) or by TPS or by RPM vs MAP (manifold absolute pressure, or vacuum reading). In TBI it's just TPS. Truck will go into ECM ignition control at 400 RPM, but will not go into full feedback until CTS (coolant temperature sensor) shows engine up to temp. At start up O2 is ignored because the exhaust is not hot enough and you want the eninge a little rich.
Fuel or ignition.

[ChevyTech]

D13 already replied to this air flow subject when I was writing this reply, but here is what I was writing.

I did a little more reading on TBI my self, am i correct in understanding that the butter fly valves (much like a carb) would act like an air flow sensor. not in the fact that they meter airflow, but more as in the throttle position sensor. more air= more fuel? in this case that would eliminate any mafs, in which i could understand.

The TBI system computer uses the Throttle position reading, RPM signal, and MAP sensor reading to calculate the air flow. The TBI trucks have no Mass air flow sensor and mass air flow sensors are not the most rugged part. In my opinion, the TBI system is the most rugged and easiest to work on fuel injection system.

which is also how you guys came up with it being a fuel delivery problem. so out of the two fuel pump sensors, if it cranks over and starts for a few rotations, that would eliminate the acc. sensor and pin the problem on either the oil pressure sensor or the ecu? or possibly a bad connection between the two.

There is much misunderstanding any many people get in arguments over this issue on some message boards.

There are two circuits that supply power to the fuel pump.

When the engine has oil pressure a redundant circuit supplies power to the fuel pump. This circuit goes through an oil pressure sensor/sender. On a 1987 this circuit goes through an oil pressure switch/sensor by the distributor.


The truck has a fuel pump relay that supplies power to the fuel pump when:
- Anytime the ECM receives a distributor reference pulse. (Engine running / engine cranking)
- The key is turned on engine not cranking – relay applied for 2 to 20 seconds (most trucks 2 seconds), depending on emissions equipment. Trucks over 8500 GVW and some other vehicles have and electronic Fuel timer module that allows the pump to run up to 20 seconds.
The key must be turned off for a certain amount of time before it will prime the system again when turned back on.

Driving down the road either circuit could fail and there would still be power supplied to the fuel pump.

If you remove the fuel pump relay, or it does not work, the engine will have to crank until it gets oil pressure, and then the fuel pump will get power.

If you pull the fuel pump relay while the engine is running it should keep running. If it dies the oil sender or its wiring is bad or the engine lacks oil pressure.

If you unplug the oil sender while it is running it should keep running also. If it dies the fuel pump relay or it circuit is bad.

also i read that any modifications done (ie. cold air or headers) wouldnt be imeadiatly rectified with the TBI because it is a learning system. once the ecu has a chance to process new info over a period of time, it would slowly make changes for the mods. thats one of the reasons they make after market chips for them?

Page down at this link to the ”Block Learn” to read about fuel tables and how they adjust
http://www.customefis.com/GMEFI.html

There is a long (Slow) “trim" adjust of fuel, and a short (faster) adjustment of the fuel mixture. Two separate “tables”.

Keep in mind there is a limit to hard far the system can move the mixture to compensate for changes. That being said, there are many thing you can do to make the system work with modification you want to do.

I am much more likely to answer a question if it is about a TBI system. It is a great system in many ways. It is much easier to see what is going on compared to port fuel injection. You can actually is the fuel, so you know if it is getting injected or not. It is a very tough system and some TBI trucks go well over 200,000 miles with very little or no trouble or repairs.

If you do modification to TBI trucks first realize there are different types of EGR systems that look nearly the same but some are sensitive to exhaust system modifications.

The fuel pump which is in the gas tank of TBI trucks usually last between 80.000 and 150,000 miles, but are bigger job to change then some people like. I would NEVER install anything but a GM original equipment fuel pump in one of these trucks. The failure rate of aftermarket pumps is staggering.

[James McClure]

Chevytech just about nails this one. I spent over 30 years in the business and don't know as much as he does. I'm sure he has seen indications of an "overheat" condition at the fuel pump electrical connection inside the tank too. Doesen't have anything to do with this thread though but something anyone with TBI or an "in tank" fuel pump should be aware of. I've never seen one go up in smoke, but who knows what could happen if that circut started to arc, right? jim

[D13]

The fuel mix in the tank is too rich to explode. Gas is not particularly flameable, but the vapors are flameable and explosive.

It gets back to the first question a good tech should ask. How did it run before the problem? What changed to cause the problem? Was it ever running good? Why did you swap all the parts? Did it run OK until the ignition tuneup? More info gets us closer.

If it ran good until you did the tuneup I would take a hard look at the ignition system. Bad cap or wire that backfeeds the module is a killer.

[Captkaos]

Another thing to check is to close off the return line and see if it makes a difference. You can also check the fuel spray with a timing light....

I had one doing this and the problem was the regulator in the TBI, the spring was shot.