Surging idle and running lean

[mclark]

Hey guys any ideas whats causing my 1994 Sho auto to surge at idle and run lean . Pulled codes:121, 136, 327, 411, and 412. Replaced egr valve twice , dpf sensor twice, iac twice, cleaned nostril ports, thanks.

 

[luigisho]

Ok so it is hunting to get back to non-lean condition. IF we assume EGR/dpfe/IAC are not the Causes?
I would look for Vacuum leak, MAF malfunction, Throttle position sensor malfunction, Injector malfunction??

What other issues are/were you having?

 

[mclark]

Switched back to stock maf, changed tps same results, whats the best way to check for vac leaks besides spraying carb cleaner in which I've done several times with no findings,

 

[luigisho]

that's the usual procedure. I used propane sometimes around all vacuum lines and connections. Have you looked at computer swap? Check fuel pressure?

 

[mclark]

I was thinking computer swap being my next option,

 

[luigisho]

with out an obvious component and vacuum leak it starts to get narrow. I wonder if an injector is not working correctly/clogged or there are injector drivers in the computer to control open/close signal. Problems on these things are getting more complicated with age and harder to help diagnose. It often drags on longer and you never know what previous issues were and what's been done, what's been checked etc.
Keep checking back in so we can follow to the resolution..even if that means killing it with fire

 

[rubydist]

121 = throttle position sensor out of range
136 = front oxy sensor not switching - lean
327 = egr feedback signal is low
411 = iac not controlling idle properly - high
412 = iac not controlling idle properly - low

You should attack these in order from lowest number to highest number, as the lower numbers are priority. That means the first thing to correct is one you have not yet touched - the throttle position sensor. The pcm is going to struggle to control idle speed if it doesn't know what the throttle position is. It may be that the sensor is bad, or it may be a bad connector or wiring to it, but that code needs to go away before you are going to have success with fixing the 411/412 codes or the idle surging.

The second thing to attack is the front oxy sensor. Most likely it needs replaced, but it could be a serious vacuum leak on the front bank of the engine. A vacuum leak would also contribute to the pcm struggling to control idle speed. These engines are now old enough that there are likely cracked/broken vacuum lines to hunt down and replace.

On these engines, the 327 code is generally not related to the egr valve or the dpfe sensor, but rather to the ports that measure egr function getting plugged up so the sensor is not able to tell if the egr is working or not. There are threads here on the forum that describe how to clean those ports. Since you say you cleaned those with no effect, most likely they were not cleaned properly.

 

[mclark]

Thanks for the responses, tps has been replaced twice with same results . While sitting here discussing the issues i remembered this same issue happened many many years ago in which i took the car to a mechanic and after testing and replacing the exact same parts with no fix he stated that the computer was possibly bad. Replaced the computer car ran fine, replacing it would probably fix the issue. The ports has been cleaned , just to make sure the intake was removed to clean ports and runners.

 

[luigisho]

If it was me I would go for it. Even if it is not the issue--it is a sellable item and good to have an extra if you can afford it. I have at least 2 mtx computers in a box somewhere. Not sure where (2 moves)

 

[mclark]

Hey guys just an update, received new computer today, installed it late this evening after work, for the first time in a while the 94 auto Sho idled correctly, no high idle and no surging at idle, so that was a plus, so far so good . Did not drive it had to take care of other business, but most definitely will test drive it tomorrow if its the Lords will, I will update as well .

 

[mclark]

Test drive was good, so far so good with no idling problems,