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TBI 350 loping like it has a big cam and stalling

J

Jorwood

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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 now 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 since I reinstalled the IAC; 33 - MAP (manifold absolute pressure) sensor circuit high (low vacuum)
 
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...
 
I had an issue like this. The egr solenoid was bad. Not the valve itself, but the black box that controls it
 
Truck had been running alright recently. I drove it for a few weeks without the Egr hose connected and decided to reconnect it. Since it has actually been running much better. Still running rough at idle but not loping while under light load/coasting anymore. However recently it started to smell like it was running rich. And then today it was unbearable inside the cab. The rich smell certainly seems to be coming from the engine.
 
If it has a fuel pressure regulator ,the diaphram in it may have failed,that will let it flood,and also create a vacuum leak..make it run very rich,lope,stall at idle--..but not all years used one..
 
Thanks for that tip. How would I know if 1990's had a fuel pressure reg?
 
Well I've learned that my EGR valve is good. However the truck still runs rough as hell. Although when I manually depress the EGR it starts to run smoother and then gets much rougher... Not sure what this means. A buddy and I ran live diagnostics on it while it was running and established that the fuel pressure was correct. As well as that the O2 sensor was functioning properly. Not sure where to go from here. He is a full on mechanic and told me to temporarily block off the EGR port so that he could run diagnostics again. Not sure how to do that or what he's looking for.
 
Follow the EGR vac line from teh egr to the solenoid . Plug the vac line coming OUT Of the tbi, into the EGR solenoid.

Mine was bad.
 
Also forgot to mention that the rich gas/fumes deal seams intermittent. It's not as bad or almost unnoticeable while idling sometimes.
 
Well it turns out that my spark plug wires were resting on the manifolds and arching. Pulling them away made an instant difference in power and sound. I also noticed my engine temp sensor wasn't plugged in. Upon reinstalling I noticed that my truck won't heat over 107-109 and the idle won't come down... and the truck seems to still smell like it's running rich. I found after more testing that my O2 sensor is very lazy after viewing it again on the graph while running, which may have a large part in why it's running so rich. Gonna install new plugs and see how that helps.
 
You may need a new 195 degree thermostat too,if the engine runs too cool due to one that hangs open it will run rich,thinking its still not warmed up..

The EGR will make the engine speed up some if you open it manually--the valve is basically a controlled vacuum leak that sucks exhaust gases into the intake manifold at a metered rate,controlled by engine temparature,and the EGR solenoid..when it opens the fuel mixture leans out,which tends to make the engine speed up a bit..
 
Did you guys watch the MAP with the scan tool? Sometimes a little work over on the pins in the connector will cure intermittent MAP issues....could be a bad MAP sensor also.

I have never seen actual low vac (vac leaks or hose not connected to map) cause a map code on tbi.
 
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