350 TBI weird idle and deceleration issues

[Daveo91Burb]

Getting some weird symptoms while decelerating in gear (compression braking) that now seem to be spilling into some idle issues after it happens. While decelerating I can hear and feel pulsing or surging. At first I could hear it out the exhaust, but now I can feel it with more, than less, engine braking. The cycle lasts about 1-2 seconds total and it' s very consistent. Today, after coming off the freeway and then driving through town with a bunch of stoplights it started idling very poorly - cutting out and feeling like it was going to die, but it never did. Acceleration and cruise feel pretty normal. After driving some more idle seemed to go back to normal. IAC valve? EGR? TPS? Something else? I have not done any diagnostics yet. Check engine light is not on, but not 100% sure it works since I've only had rig a few months. It's a '91 suburban 4x4, 3/4 ton, 350 TBI, more or less stock but it has been chipped and it does not have a cat.

 

[89Suburban]

Injector taking a ****? Sounds like a nice Burb.

 

[Daveo91Burb]

Thanks! Don't think it's an injector problem though since it only happens at idle and decel? I'm thinking I might just pull the TB and give it a good cleaning and see if that helps. Might add a spacer while I'm at it. Was just hoping my symptoms rang a bell with someone and that they knew of an easy fix. Guess there is no such thing with TBI!

 

[Jims86]

Thanks! Don't think it's an injector problem though since it only happens at idle and decel? I'm thinking I might just pull the TB and give it a good cleaning and see if that helps. Might add a spacer while I'm at it. Was just hoping my symptoms rang a bell with someone and that they knew of an easy fix. Guess there is no such thing with TBI!

If the check engine light does not illuminate when you turn the key to "on", the bulb is likely to be toast. I would start there, and also inspect the wiring/or any plugs on the Vehicle speed sensor, which is located in the tailhousing of the transfer case. Inspect the wiring at the IAC as well. Not going to talk you out of the Throttle Body Cleaning.

 

[Daveo91Burb]

If the check engine light does not illuminate when you turn the key to "on", the bulb is likely to be toast. I would start there, and also inspect the wiring/or any plugs on the Vehicle speed sensor, which is located in the tailhousing of the transfer case. Inspect the wiring at the IAC as well. Not going to talk you out of the Throttle Body Cleaning.

Thanks! Check Engine light (actually Service Engine Soon) comes on at Startup and then goes out so I guess the bulb is working OK. Can these rigs still throw codes w/o the light being on? I checked the plug at the VSS sender and it looks OK. If that actually is the culprit, would the speedo still work? (Speedo works fine). I'll proceed with TB cleaning and take a look at IAC then. This is not a huge problem - rig is still quite drive-able (although it may be getting worse) but I do want to figure it out. Last night I also tried actuating the EGR plunger by hand (engine off) and it moves freely. I'll try doing the same thing with engine running in the next few days and see what it does to engine sound.

 

[Jims86]

Thanks! Check Engine light (actually Service Engine Soon) comes on at Startup and then goes out so I guess the bulb is working OK. Can these rigs still throw codes w/o the light being on? I checked the plug at the VSS sender and it looks OK. If that actually is the culprit, would the speedo still work? (Speedo works fine). I'll proceed with TB cleaning and take a look at IAC then. This is not a huge problem - rig is still quite drive-able (although it may be getting worse) but I do want to figure it out. Last night I also tried actuating the EGR plunger by hand (engine off) and it moves freely. I'll try doing the same thing with engine running in the next few days and see what it does to engine sound.

Ok, Cool.
The ECM activates the light on any trouble code to report, however, you will only get it for a troubled sensor, OR IAC. you will not get codes for Fuel issues, like pressure, fuel pump, bad injector/spark misfire.
Sensors do get lazy though, and until they meet a pre set threshold in the ECM, when it sets the code, and gives you the light. So, that said, you could have a sensor or two on their way out, causing little problems, but have not yet met the requirements to set the code.
I would go ahead and clean the Throttle body, maybe even put a rebuild kit in it. Check the throttle shaft for play,,like on Carb. Soak or send out the injectors for cleaning.
Be aware, that there are two types of EGR valves used on these trucks. one holds vacuum only when there is exhaust pressure while the engine is running, which is the Negative type.
Then there is the positive, which you can apply vacuum, and it holds without exh. pressure. A Lot of trucks have the Negative type, but most OEM valves have been replaced by now, and its hard to tell which you have, because the remans don't always have the "N" or "P" identifiers stamped on the valve. If you still have your emissions diagram, it should say either EGR(positive) or BPEGR( Negative)... but keep the question in your mind, "did the last guy use the right replacement.
I would start the journey with inspecting for Vac leaks...vac lines, gaskets. TBI is really sensitive to vac leaks, due to the MAP being the Major sensor that is used for Every operation of the system, save for the knock sensor circuit. It will cause the stalling at idele, and will also effect decel.

 

[scottybaccus]

Check fuel pressure. a buddy had a similar issue recently. He went after the fuel pump, but it was also a weak spring in the pressure regulator. That's an easy fix, but very often overlooked. The spring dies due to age. Replace it and then test pressure again to evaluate the pump.

 

[Jims86]

Throw a new fuel filter on while you're at it.

 

[Daveo91Burb]

Thanks, guys. In the process of ordering transdapt spacer and Delco rebuild kits for TB and fuel regulator. Will do fuel filter too. Will keep an eye out for possible vac leaks or EGR issues.


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[Daveo91Burb]

Well this may end up being a lot simpler than I thought: I found today that the pcv pipe was not connected at all to the TB. There is supposed to be a 90 degree bend in the hose where it hooks to the TB - that piece is just gone. So I guess this would be like a massive vacuum leak? Hopefully I did not screw anything up since it would probably be running pretty lean with the PCV not hooked up? If I put my thumb over the port on the TB the engine would quit - is this normal? Seems like on my carbed vehicles if you did not want to use a port on the carb you just put a plug over it and everything was cool.

Will get a new PCV valve and 90 degree bend tomorrow and see how she runs after that....

 

[89Suburban]

Holy mackerel