Start and Running Problems 1988 R20 5.7

[Rusty Shackleford]

Looking for some things I could look into which might solve this problem I am having. Have searched and found some threads but they sound a bit different than my situation. Previously, the truck would start with no problem but would run rough at low speed. If I drove it to the end of the block then shut it off for about 15 to 30 seconds, it would fire up and run fine. This happened for a while. Now it will start up right away but go into rough idle and stall. Upon re-start, it won't fire unless I press the gas pedal to the floor. Higher idle speed is smooth. Check engine light came on and i found a 33 code (MAP Sensor). Replaced the sensor, air filter, and gas filter. Same problem as before. I took the air cleaner lid off and (engine off) a white vapour was coming out of the TBI from below the injector (passenger side) and a smell of gas. When I get it started it runs rough before it stalls. I will pull the plugs to see if there's something there. Any suggestions appreciated. Thanks, in advance.

 

[Tank6x2]

Sounds like a intermittent fuel pump issue… but the white vapour throws me… how old the pump?

 

[Rusty Shackleford]

The fuel pump is the original from the factory (1987).

 

[Curt]

I agree.The pump.I would change the fuel filter first.

If that don’t work,at least try and run most fuel out before dropping tank.The sock on the sending unit gets really nasty over the years.

But,it could also be a injection nozzle.Runs rough at idle,but better at high idle.Kinda sounds like it’s flooding.,IE the vapor.

I’m sure someone else here has had the exact issue.Just keep the thread alive so it’s seen.

 

[Tank6x2]

I agree.The pump.I would change the fuel filter first.

If that don’t work,at least try and run most fuel out before dropping tank.The sock on the sending unit gets really nasty over the years.

But,it could also be a injection nozzle.Runs rough at idle,but better at high idle.Kinda sounds like it’s flooding.,IE the vapor.

I’m sure someone else here has had the exact issue.Just keep the thread alive so it’s seen.

Believe he did the filter

 

[Bextreme04]

Don't just throw parts at it. That will get expensive fast on a TBI. I would recommend you go to the resource library on this forum and download the emissions and drivability manual for your vehicle. There will be a troubleshooting flowchart for your symptom. It will most likely have you check a bunch of stuff including MAP sensor voltages, TPS sensor voltages and readings, actual fuel pressure, actual manifold vacuum, EGR voltages, and perform an inspection for vacuum leaks. I have seen where a bad TPS connector can cause this issue, bad ECM, bad EGR valve, low fuel pressure.... etc.

You could throw all the parts you want at it and if it turns out the connector was bad and not allowing decent contact.... it still wouldn't fix the problem even if you dropped a new engine in.

 

[Rusty Shackleford]

Thanks for the info. Bextreme04: Could you direct me to where I can find the emissions and drivability manual? I can't seem to see it.

 

[Bextreme04]

Thanks for the info. Bextreme04: Could you direct me to where I can find the emissions and drivability manual? I can't seem to see it.

https://www.gmsquarebody.com/threads/1973-1991-service-manuals.14499/

Scroll down and click on the 1988 "Fuel and emissions including drivability" link

 

[Bextreme04]

Thanks for the info. Bextreme04: Could you direct me to where I can find the emissions and drivability manual? I can't seem to see it.

Also, to actually tag someone so that they get an alert that they have been mentioned in a comment... you need to either quote one of their prior replies like I just did, or when you type their names like you did above you need to start it with the "@" symbol. Like this @Rusty Shackleford. Once you start typing the first few letters of their screenname after the @ symbol it will show a list of screennames that match and you can just click on the one you are looking for

 

[fast 99]

It would be much easier with a scan tool. Just did a search on e bay there are several for sale. Looks like about $200-250 will buy a working "red brick" snap on scanner. About the same as one diagnostic trip to a shop. If you proceed without one and check systems with a volt/ohm meter, be sure to check every connector for broken wires, corroded pins and pins backed out. Also check grounds.

If you think it is the pump, try hitting the bottom of the fuel tank with a golf club or bat when it is running poor. It will shake the pump and might run better for a very short time.

I think that application has an EGR valve. With it running at idle take a finger, reach inside the EGR housing, pull up on the plunger. It should die or run much worse. If no change, the valve is leaking/bad.

 

[Bextreme04]

Or if you have an android phone or tablet that can run ALDLdroid app or a laptop that you can connect, you can just get an ALDL to USB/Bluetooth tool like this one:
https://www.ebay.com/itm/114869457343?hash=item1abec0d1bf:g:MY0AAMXQMmJROrFI

I agree with Fast99 that it is MUCH easier to troubleshoot and EFI system if you can have live access to what the ECU is actually reading.

 

[Jwernatl]

Go here - you fix anything with these manuals especially the ‘drive ability manuals’

http://www.mediafire.com/download/q...Fuel_and_Emissions_Including_Driveability.pdf

 

[evorandyp]

You mentioned a white vapor rising under one injector. That would tell me that the injector may be leaking or stuck open. If the one injector is dumping to much fuel that could cause the symptoms you described.

 

[Rusty Shackleford]

An update. I've checked the EGR valve. Holds vacuum. Took it out. Very limited carbon build up. Checked vacuum hoses. all seem to hold vacuum. Injectors seem to have a nice spray on them. Started up fine. Drove a block and it started to run rough. Stopped engine. Hard start and rough idle, then idle smoothed out and it ran fine all the way home. Will check for vacuum leak at intake manifold next and delve more into the drivability manuals.

 

[Rusty Shackleford]

I've read varying opinions on OBD1 and OBD2 systems. Will connecting an OBD2 scanner to a OBD1 vehicle via 12 pin and 16 pin cable adapter provide OBD2 type data from your OBD1 vehicle? Some people on-line say "Yea,"and others say "Nay." JUst thought I'd ask to help me make a decision about purchasing an adaptor cable (or maybe an OBD1 scanner). Some say you can only read codes with a OBD2 scanner on an OBD1 vehicle. There are a lot of adpators cables available so I'm leaning towards they will work. Thanks