What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Surging idle in Park only........

Burbdood 74

Burbdood 74

Giver of not a single damn...
Joined
Jan 12, 2008
Posts
3,208
Reaction score
17
Location
Billings Montana
Hey all.

I have a problem that is driving me up the wall. 88 Suburban TBI 350.....

I have a surging idle but only in Park.

It idles fine while you're sitting at a light or something like that in drive, but the minute you put it into Park, it surges pretty badly. I have looked and looked for vacuum leaks and I cannot find one anywhere. I replaced the fuel filter and I have a nice spray pattern from the injectors. I have also cleaned the IAC and the EGR valve and replaced the MAP sensor. I have no idea what to check next.

The thing that gets me is the fact it only surges in Park/neutral and not in gear. Could it be my Park/Neutral switch going out on me?

PLEASE HELP ME!! This is making me crazy.
 
well the only thing i can add right now that you may or may not already know is that neutral and park put no load on the motor..

I remember having this problem long ago and it was like neutral dropping the trans every time..

i dont think it was a leak because either the truck would want to rev up in every gear on its own. your TBI is computer controlled to a point.

I dont think its a park neutral switch because obviously the tranny switches gears fine.

I personally think (just an opinion) that your computer is adding more fuel in these gear selections and is bumping up the idle to compensate for too much oxygen..


Does it want to drive itself in drive or in gear?
 
Yeah, when you are in gear it will drive itself just fine if you lay off the throttle and just let it putt from a stop. It runs great running down the road and on the highway too.
 
Well I changed the EGR with the one listed above and it runs a lot better....but I believe my problem is in the fuel pump.

When you start it, it runs smooth for a bit in PARK, then it starts to surge. If you give it a little gas, it smoothes out and eventually starts to surge again. The second you drop it into DRIVE, it smoothes out and you don't get a single surge or hiccup out of it until you put it back into PARK. :dunno:


Thoughts?.......
 
From what I've seen, fuel pump issues don't normally act like that...if anything, once under load, they stall.

Just in general, if you've got an Autozone out your way, they will loan you many diagnostic tools, and from what I could tell, it looks like their fuel pressure test kit includes a TBI adapter, so that is something you could do for "free".
 
Sounds like the IAC. I know you cleaned it, but I see this a lot in electronic control systems where the mechanical part is not responding right.
In park or neutral, the IAC is in a certain position to maintain the correct idle speed. Since things change, it is constantly moving in small amounts to keep that speed constant.

If the RPMs start to drop off slightly, the IAC tries to open to compensate. If it sticks, the computer tries to open it more as the RPMs continue to drop.
Then when it responds, it overshoots.

When there is a load, it is holding at a different place, and it does not stick there.

The other thing, is the base idle speed. Have you set that? Its critical too. It is the speed the engine runs at when the IAC is out of the picture.

There is a procedure here for setting that, but I've got to run or I would look for it.
 
Yeah.....after reading the link to the thread you gave, that's way, way, way above my pay grade. My wife has family in Spokane so maybe I will insist we pay them a visit....with a detour to your side of the border. :whistle:
 
If its cycling IAC probably working, EGR,TPS, Vac leak, need to pull codes.
These things can trick you. In gear ok, what changes?

When you say idles ok for a bit when first started, do you mean long enough to come up to normal temp.
Open loop\Closed loop. ECT involved.

IMHO
 
IAC logic is completely differant in P/N then drive. There is one wire from P/N switch to ECM that is grounded in P/N and open in drive like a manual trans. It changes how the ECM handles things.

I agree with checking mechanical like vacuum, codes etc... but what is happening to him I have never dealt with, and I have dealt with a lot! So other then a mechincal issue I would have to see data to find what is wrong.

Surging number one issue is a vacuum leak, but still can't explain why differant in drive?

Data would show if IAC count is way high! This could cause this. As a TBI wears the throttle blades close and there's not enough air going over the plates to mix the air and fuel, so fuel spills/pours in and air comes through IAC... but then I can't explain why it would be so good in drive? Just a weird issue I can't expalin nor have dealt with... you could turn in min air screw 1/4 turn to see, it would not hurt anything, but again without data it's just an educated guess.

IAC are fairly cheap, you could throw one on? I just don't like to throw money and parts at something when you can see data or run some diagnostic test to prove.

See, I'm guessing, you could end up replacing everthing and still not fix the issue!
 
blazer74 said:
If its cycling IAC probably working, EGR,TPS, Vac leak, need to pull codes.
These things can trick you. In gear ok, what changes?

When you say idles ok for a bit when first started, do you mean long enough to come up to normal temp.
Open loop\Closed loop. ECT involved.

IMHO
Click to expand...



When you first start the Sub, it idles good for about a minute or so and then it starts to surge slightly. You can drop it into gear and it smoothes out completely and idles fine. After you drive it around a bit up to operating temp, you can put it into PARK and it surges badly almost cutting out.

I have replaced the following in the last 2 years with this thing:

IAC
Temp sensor
O2 sensor
rebuilt the TBI
TBI base gasket
MAP sensor
TPS
Distributor
EGR vacuum solenoid
EGR
EST
Fuel pump and filter
Intake manifold gaskets

I cannot find a single vacuum leak anywhere and I have no codes to speak of except for a code 44 once in a blue moon. The last time I had that code was 3 months ago. It runs down the road fine and it has awesome power but I do have some bucking and surging sometimes at part throttle under 30 mph.

That is what leads me to think that maybe my fuel pump might be lacking the proper pressure but at this point I am completely stumped.
 
Chasing a gremlin, If u can find someone with a code scanner that can read live data during the problem might shed some light. The surging at cruise leads me back to EGR. Disconnect it see what happens. Pull your ECT sensor connector off see if there is any change.
Did u use carb cleaner to search for Vac leaks, Had one up under manifold on my ranger I couldn't find until I used the tube and sprayed up under.

I don't remember if u mentioned if it does it in neutral also.
Retrace steps for probably the fifth time. Re seat connectors, tighten TB etc.

Those throttle body's do crack sometimes.
EGR keeps coming back at me.
U might induce codes if u pull connectors with power on.
 
Yeah but he just gave us a clue in error code 44 which is O2 sensor lean! So his theory of fuel is valid! Check fuel pressure!

It's an old truck and I've seen the screen rot and pump suck up the fibers. At that age it should have been replaced. The new filters are differant.

Again still guessing. If you do make it by here I can run data and see why. Could fix the chip while your here... :whistle:
 
Another thing and I don't know if it's related or a seperate issue is that when I go to put gas in it, I get a lot of pressure escaping from the tank when I crack open the cap. I never had that before I had the fuel pump replaced.

I've often wondered what the asshat that replaced it did to the poor thing. Could he have put a vent line on wrong or pinched it off and that's causing me some of the issues?

Oh crap, I might as well drop the tank and see what the dood did to my Poor.Old.Sub.
 
It's probably not pressure but vacuum. Leave the cap cracked open a little and check. But this is normal to a degree.

Check fuel pressure, no reason to drop tank... yet... you can't see pressure!
 
Looks like it's time to use the new shiny pressure guage I just bought...:D


Now if it would just get over 12* I could get excited about working on it.
 
You must log in or register to reply here.

Latest Posts

Top Bottom