TBI 350 Loping like it's got the cam I wish it had... need help

[jorwood]

I have a 1990 K5 blazer with the TBI 350/700R with 174k miles. Recently the car started surging/running rough at low RPM to the point that I stopped using overdrive. I also noticed it being rough just idling forward. I had been getting a code since I had redid the head gaskets intermittently and finally decided to check on it. It turned out I never reinstalled the vacuum line to the EGR valve. Upon reinstallation it seemed to fix everything. Then the motor started loping like it had a big cam with a low idle while I was coasting to a stop. I noticed it doing this as lightly drove forward in a parking lot as well. It never does this just at idle, only while moving. It's also fairly inconsistant as I can't make it do it everytime. I suspect it has something to do with the air idle control valve. I took it off but it doesn't look particularly dirty. I'm not sure how to clean/test it and I don't want to just throw $50 at a new one until further diagnosis... Also I has since started running rough at low RPM again and it doesn't sound very healthy...

Well I just reinstalled the IAC and it won't even idle without a little throttle and sometimes it will idle but BARELY I mean I couldn't even hear it at first. It does the loping thing even just sitting now but sometimes it's just smooth. On the drive home it felt like power was starting to cut out... It did throw a code; 33 - MAP (manifold absolute pressure) sensor circuit high (low vacuum)

Thanks

Jordan

 

[jorwood]

I have no clue why it's running even worse after just removing and reinstalling the IAC...

 

[jorwood]

After researching further it seems to be running worse because the IAC needs to be reset... I've looked online and I can't get a clear idea of what the procedure is... I've tried a few and it still won't idle unless I'm giving it gas... one thing I'll mention is that when I took it off I tried running the engine without the IAC installed in the TB but still plugged in to see if it was moving. After a few seconds I promptly turned it off due to a very loud sucking sound that I feared might be damaging. Anyways this my work truck and I am desperate to get it driving or at least idling again...

 

[Jarhead79]

You need to reset the IAC, put a paper clip in the obd port in the a and b slots, on mine it's the farthest right upper tw, then turn acc on, should hear clicking noise in the IAC, it's fully closed now. Then unplug it, then remove jumper pin and start it, then plug in the IAC.

 

[jorwood]

Thank you for the reply! I did as you said and heard the IAC clicking but the IAC motor never stopped... seems like somethings a foul.

 

[Jarhead79]

It'll continue to click, that means it's as far forward as it can go, unplug it then shut off the motor, then plug it back in and restart the motor

 

[1987 GMC Jimmy]

Is it any better after the reset?

 

[jorwood]

It didnt make a difference... still wouldnt idle.

 

[1987 GMC Jimmy]

It didnt make a difference... still wouldnt idle.

This condition happened to me once. I think it happens to every TBI owner at some point or another. It kinda sounds like a MAP sensor issue. Check the vacuum line that goes into the MAP sensor, and if it's okay, unplug it and see how it runs. If the sensor is bad, it should idle okay. Rich but okay. Oh! Make sure you didn't melt the MAP vacuum line. That happened to me when I replaced the EGR, and that's what made me have the issue you're having.

 

[jorwood]

This condition happened to me once. I think it happens to every TBI owner at some point or another. It kinda sounds like a MAP sensor issue. Check the vacuum line that goes into the MAP sensor, and if it's okay, unplug it and see how it runs. If the sensor is bad, it should idle okay. Rich but okay. Oh! Make sure you didn't melt the MAP vacuum line. That happened to me when I replaced the EGR, and that's what made me have the issue you're having.

I wanted that to work so badly... alas all the vacuum lines look good. Unplugging the map just made it run like hell. Still won't idle.

I am pulling my hair out wondering how it could have run so much better before I reinstalled the EGR vacuum line... And now it's worse still after testing the IAC...

I appreciate you guys trying to help! I have a new IAC sitting here... just awaiting more comments before pulling the $50 plug.

 

[1987 GMC Jimmy]

I wanted that to work so badly... alas all the vacuum lines look good. Unplugging the map just made it run like hell. Still won't idle.

I am pulling my hair out wondering how it could have run so much better before I reinstalled the EGR vacuum line... And now it's worse still after testing the IAC...

I appreciate you guys trying to help! I have a new IAC sitting here... just awaiting more comments before pulling the $50 plug.

We'll get him figured out. I guess it's not the MAP sensor. Have you looked at your injector spray? I've experimented with my TBI, and it'll run like you say if only one injector is working. Just to be safe, make sure you have a nice conical spray coming from both injectors. I wish we knew your fuel pressure. There's just a bunch of stuff to check.

 

[jorwood]

is that something I would see just by looking into the TB while it's running? or do I need to pull somethings off? It still drives ok, but it doesnt feel very powerful... but that could just be how these things are lol.

 

[RetroC10Sport]

Checked timing lately?

 

[Georgeb]

I would retrace your steps. Is the egr sticking?
Try unplugging it and plugging the hose.
Double check all the other vacuum hoses are routed correctly.

 

[1987 GMC Jimmy]

is that something I would see just by looking into the TB while it's running? or do I need to pull somethings off? It still drives ok, but it doesnt feel very powerful... but that could just be how these things are lol.

Yeah, you'd watch it while it's running and make sure that it's spraying well. If it's messing up, it would spray too little. I'd go back and check your timing, too.