[jeebalow]

Hey guys. So I have a 95 TBI 350 with about 190K miles on it. This started as a surging while driving at constant rpms. Very noticeable in second gear at about 25 mph. The truck has a NV3500 so the whole truck would surge.

I replaced IAC, CTS, TPS, ETS, EGR valve, EGR solenoid, FPR, Injectors, fuel filter, plugs, wires, Cap, rotor, pickup coil in dizzy, PCV valve, and got a tune from TBICHIPS. Somewhere along the line the surging stopped while driving and started only happening at idle. The RPM's would go up and down and then go back to being smooth and come back later. It got worse over time and now it when that happens the RPM's go up to about 1200 max and down to almost dead a few times and eventually it can't recover and just dies.

If I tap the gas while the fluctuating is happening it goes away for about 10-20 seconds. So I wasn't suspicious of fuel pressure, then later checked it and I have a steady 10psi from the moment I turn the key until a few seconds after it dies so the fuel pump is good. It runs strong and is fine until I idle.

This happens only when the engine is warm and last time I checked it never went into closed loop. After about 2 years I gave in and took it to the dealer.... they had it for 3 entire days and could not find anything wrong!!! I am running out of ideas and hope any help is greatly appreciated.

If you're still reading this, have a TB spacer, K&N CAI, Shorty headers, stock y pipe, flowmaster, and the TBICHIPS tune if that matters any. Thanks again

I should add that when revving the engine in neutral at a constant 2500 rpms it will sound nice and constant and then sound like for a split second power was lost. This is so short that the rpm needle hardly drops any when it's back up to the constant 2500 rpm. This happens every 2-3 seconds while holding the revs constant and the truck is warm.

 

[SuperStepside]

ETS?

What about the O2 sensor? Once the engine is warmed up then it relies on the O2 sensor for air/fuel ratio.

A rough idle usually means a vacuum leak. I would check for those especially vacuum lines.

Does it idle better if you add an alternate fuel source? Like brake cleaner, carb cleaner, or propane?

 

[jeebalow]

I did note ETS in the list in the OP I forgot to say O2 but that is new also. And if you keep reading it also says that last time I checked it didn't go into closed loop. I still need to verify that though. Just don't remember if fast blinking or slow blinking is closed loop?

 

[jeebalow]

Forgot to mention I have checked for vacuum leaks time and time again, and I haven't added an alternate fuel source. I'll try tomorrow. Can you explain what a better idle with another fuel means?

 

[SuperStepside]

I meant what does ETS stand for? Never heard of it.

You add another fuel source if you suspect that you have a vacuum leak. When theres a vacuum leak the engine is running lean and rough. By adding fuel you'll richen the mixture and compensate for the vacuum leak. The engine will then run smooth.

 

[jeebalow]

Ooh ok gotcha, ETS=engine temp sensor. It's what controls the gauges, and is on the driver side head IIRC. It's probably irrelevant to any problems but I replaced t anyway lol.

And I get the alternate fuel now, I'll check. Thanks!

 

[jeebalow]

Checked the alternate fuel thing, used chemtool carb cleaner and no luck. The truck sounded bogged down and wanted to die when I sprayed it in the throttle body. It didn't prevent the fluctuating idle or stop the truck from dying.

The only ground I saw on the intake was the one on the thermostat housing and that was cleaned at the dealer. Is there any other grounds on the intake?

 

[MeanGreenZ71]

I had a slightly high idle problem and a rough idle at times. I rebuilt my throttle body with a kit from Ebay and it cured both problems. I guess I had a vac leak somewhere in the throttle body or in the gasket underneath the throttle body. I tihnk the kit cost me $35 or something and the truck ran alot better afterward. With 160K on the TB it was time for a rebuild and good cleaning.

 

[jeebalow]

The throttle body has been rebuilt with new injectors, fpr, and orings. The idle is not high. It fluctuates and sometimes it cant recover from the drop in rpms and dies. I have checked for leaks and there is none that I have been able to find.

 

[MeanGreenZ71]

Im just sayting that a vacuum leak can be hard to track down. I checked for one multiple times with all kinds of methods (propane, carb cleaner, etc) and never found one. It ended up being in the throttle body.

 

[jeebalow]

Oh ok, well it was rebuilt last summer as part of diagnosing this problem. Unfotunately didn't help ...:banghead:

Edit- I might grab a throttle body for the junk yard and try it on mine to see if it helps any

 

[MeanGreenZ71]

Huh IDK what to tell you. It sounds like a vac leak and the IAC cant keep up with the changing air....

 

[jeebalow]

I'll get new gaskets for in between the throttle body and intake and swap those when I swap the throttle body. Maybe I'll port/polish it also... Just don't want it to create more problems... I'll leave it for another time

 

[SuperStepside]

No no no. You tried the alternate fuel and the motor bogged down. That means that you dont have any vacuum leaks.

Do you still have the factory chip around? When I got my first chip from TBIChips it was a bad tune and the truck ran horrible.

 

[jeebalow]

Yeah I have the stock chip, but it happens with and without the chip. I bought the chip because he could maybe help tune the problem out. Brian from TBICHIPS read my datalogging and was recommending things to check. I set the timing and checked the fuel pressure as well as swapped the pickup coil in the dizzy from his suggestions.

 

[SuperStepside]

Ok I would take the distributor out and check the gear for a knife edge wear on the teeth. Was the ignition control module replaced?

Is the timing chain original? You can check the slack by taking off the distributor cap and spinning the motor over by hand. Look at the rotor while spinning the motor over. It should spin at roughly the same time you spin the motor over. If theres slack then you could spin the motor over but the rotor wont spin till the slack is out of the chain.

 

[jeebalow]

I had the distributor out to replace the pickup coil, but I don't remember how the gear was. I will keep that in mind.

Yes the timing chain is original, but can't I just advance the timing to take up the slack?

 

[jeebalow]

So update on this. I datalogged and sent the data off to Brian at TBIChips and he made another chip and sent it out to me. I installed it and per his suggestion brought timing back down to 0 degrees (I had it at 4 from the weekend before) and adjusted the throttle stop until the IAC counts were between 10-20 on a warm engine and she idled fine! So looks like that problem is now solved!