350 tbi idle speed

[Slooptin]

So I’ve got this issue with my tbi 350. Normally at idle it’s around 700 at cold start in park and it drops down to about 400 in gear and will sometimes drop and rise and even die. I’ve replaced the iac valve and done the relearn procedure and still nothing. When I unplug the iac it’s fine. Idles at about 1100 in park and 900 in drive which is high but it’s better than it dying randomly.

I know for sure I need to replace the injector cup seals because one injector is leaking out the top, but that may be unrelated to my issue.

I’m kind of stumped at where to go now aside from replacing parts. Any advice as to what to do?

I’ll probably try and check timing this weekend, but I don’t think that the issue since I’m not really having any other issues that would make me think it’s timing. I’m also aware that adjusting the idle stop screw isn’t the solution either since it will idle at 1100 with the iac unplugged

 

[gmbellew]

I believe idle is supposed to be 525 in D on a warmed up engine.

if it is bogging and idling low, I'd suspect the leaking injector may actually be a big part of the issue.

I am idling rich with occassional idle variations an occassional, almost imperceptible stumble during idle right now on my 350 tbi. I am finally getting the adapter to check fuel pressure this week to make sure that isn't too high. if that checks out, I'll likely be pulling the TBI for a refresh of gaskets/seals/Orings because some extra fuel is getting in somehow on mine.

 

[Vbb199]

Does your current exhaust have a o2?

We discussed what a tbi should be I think in another thread
I don't remember the details


To me that sounds like too much gas like a leaky injector

 

[Slooptin]

Ya it does but I’ll probably replace that so as I’m about to add a cat back in to pass emissions

 

[Slooptin]

Ok, so I replaced the o2 sensor and re-sealed the injectors and it’s still wanting to die at idle. Could there possibly be a mystery cam in here causing it to want to idle so low? I’m searching for a vacuum leak today since I hooked up a vac gauge to a manifold port and it was reading like 10-12 inhg

 

[Slooptin]

Also worth noting that I tried to reset the IAC multiple times with two different sensors and every time i end up getting a crank/no start situation

 

[gmbellew]

reset the IAC by simply turning key to run for 5 sec, then off for 10 sec, then start. that should reset it if the IAC is good.

10 or 12 inches of vac is low for a stock engine. not sure what a different cam would do, but if somebody stuffed something bad in there, it could be an issue. I am usually idling at about 19 or so on a warmed up stock engine under no acc load. you likely have a vac leak somewhere, I'd say. and not a small one.

 

[Vbb199]

Not sure where you are in relation to sea level, but here @ 700 feet on a stock tbi cam i have seen about 18-19"

 

[fast 99]

The ECM primarily uses the map and CTS to set fuel trim. It is fine-tuned by the O2 when hot. Low vacuum, ECM thinks the throttle is opened and increases fuel. The increased fuel flow is not needed or wanted at idle. Will never run right with low idle vacuum.

 

[OlWils]

I agree with Fast 99. At low vacuum the timing is also retarded. Have you checked timing with the bypass cable disconnected? It should be 0 degrees on a TBI. It is also important that the check engine lamp is working so you can see if there is a trouble code.

 

[Slooptin]

I’ve got a timing light on order so I’ll check that.

So far no vacuum leaks that I can detect. I’m aware that low vac is a bad thing so I’m trying to diagnose it.

The truck also will occasionally “pulse” at idle where I’ll put it in park, the rpm’s go back up and then bounce up and down about 2-300 rpm in park until I shut the truck off. Any ideas where that’s coming from?

 

[Slooptin]

It also has a code 34 but I suspect that's due to low vacuum and not a bad sensor/wiring, so I'm still back at trying to figure out why the vacuum is so low.

Everything is pointing me to a mystery performance cam that a PO installed.

 

[Snoots]

Low vacuum, cracked vacuum line.
Timing. I hear this a lot. Once you spend the time to set it correctly, fugetaboutoit!
It's not like, "Gee. I drove 100 miles this week. I have to reset my timing!"

 

[Slooptin]

Ok, so a little update, I replaced wires, cap, rotor & plugs. That didn't fix the issue. I went to reset the timing and noticed that the harmonic balancer must have slipped. When I try to set it to 0 degrees, it dies. It is still surging/dipping randomly at idle which I'm thinking isn't related to the slipped balancer, and still don't have a clue on.

Could a slipped balancer cause me to keep having these low idle issues?

I can get it to idle with the reference wire unplugged, but as soon as it plug it back in, it wants to idle at 350-400 rpm and basically dies when I put it in gear

 

[RacingTurtles]

A lot of what you are describing sounds very close to what I had going on.
Mine would normally cold idle at 1200 rpm, hot it would come down to 800 or so. Ran smothly.
Then I had my cyl 5 or 7 plug wire touching the exhaust so it burnt up.
It was not holding a steady idle. First it started hesitating from 800 down to 700 and back up.
It got lower and lower.
I just did a crap load of tuneup stuff this weekend ( plugs, wires, dist cap & rotor, ignition module) and it still ran like dirt.
I also hooked up my purge vac line on the tbi ( it was disconnected ) and I replaced a really badly cracked vacuum line behind the distributor ( no idea what it was going to )
Granted I did notice that my distributor is rusty, so that's gonna be my next job.

How does it run without an air filter on it?

Mine ran really nice without my old one installed ( this was after doing all the other things mentioned so it was still off the truck)
As soon as my old one went back on, it repeated the same symptoms. The next morning I got a new air filter and it seems to be doin ok.
It's now just sticking to an 800 rpm idle whether it's hot or cold. In gear it's dropping down to like 650-700 rpm but it drives pretty good now.

My old air filter did not look bad. Normally the paper element looks an off white/light tan and I noticed it had like a light black/light gray tint to it.
I would see how it behaves without the air filter first before you start doing some more work to it.