I am going to put this to the Board Because it sorta Has me stumped.

Sorta.

The car is an 88 GTA. L98, A4.

7749 ECM, Vortech S-trim, SLP headers w/ SLP runners and BBK TB.

This car used to be MAF untill I converted it to 7749. It's Been fine with the 7749 setup for the last few months.

However, now it has developed a Closed loop Idle Surge.

Surging between 600-900 RPM in Closed Loop At idle.

Using Datamaster, the RPM trace actually looks like a nice sin curve when it's doing this... its very predictable and regular.



So, I have been all over this thing with the laptop and there is nothing wrong computer wise. No Codes, and No erroneous sensor data. It's not doing anything funny.

Checked all the obvious things, and Nothing is turning up. No Vac leaks, nothing.

IAC isnt involved because 1.) IAC counts are not changing enough to cause this and 2.) it does it even with the IAC closed and unplugged.

The only time it won't surge is if I disconnect the EST Bypass. However, the ECM is holding a constant spark advance +- 2 degrees througout.

Tried a Differant module just for kicks, no change.

Tried a Differant Coil, nothin.

Pickup coil is/was new as of say 500 miles ago.

After Fiddling with the thing all day, I noticed water dripping at a rather exagerated rate from the tail pipes. After idle upon the application of throttle, a nice puff of Steam Exits from the Exhaust.

No pressureization of the Cooling system, and the Smog test Probe when Inserted in the Radiator doesnt detect any hydrocarbons, So no Dead giveaway of A blown headgasket.

Also, the oil is fine. No Milkshakes.

Have yet to get to the plugs or do a compression test, thats the next step.

I personally think it is a Blown headgasket that is causing the surge. However the people responsible for this car are reluctant to get involved in a headgasket swap without more convincing I don't blame em.

My arguement is that Since there is steam in the exhuast ( and this is after running for hours, on a very warm and dry day - this is not condensation ) That there is a problem either with the headgasket OR the intake manifold, and either way that needs to be fixed regardless of the surge.

Also, this car is blown. Stock Engines at 7 PSI are very suspect for wasted headgaskets in my book.

So, I guess my main questions are

1.) Who else thinks that Water dripping from the exhaust at a fairly accelerated rate + steam out the exhaust on acceleration Points to a blown headgasket ?

2.) Any reasonable expectation that this Blown headgasket could cause a closed Loop Idle surge?

Gracias.

 

Might be a head gasket, or an intake as you stated. Personally I'm leaning more towards the intake for some reason. I guess I'm thinking that it has an internal vacuum leak. Provided the rest of the motor is sealed up good, try sticking your finger over the PCV valve hole and see if ya get any suction.

 

Re-check the initial timing advance against what's in the chip...I've worked w/ two SD cars that got really whacky @ idle when the dist wasn't exactly where the ref angle in software was.

Aside from that, I'd pull the O2 sensor and inspect for deposits. Coolant shows up like white dots and whatnot, anything other than nice 'n tan is bad.

If it's a Bosch O2 sensor, grab a Delco and swap it out. I read Grumpy's comments on Bosch O2 sensors, and several LS1/LT1 guys have posted that weird SES codes and whatnot began after installing Bosch O2 sensors and cleared as soon as Delcos went back in.

 

had a similar problem with my '91 L98... turned out to be injectors. how many or which ones, i couldn't tell. replaced the set and all was well again and still is 25k+ miles later. i beat my head on the wall trying to figure it out, swapped distributor, coil, wires, did a compression and spark test, fuel pressure test, etc. that was all that was left, and turned out to be the culprit. and it happened less than 2k miles after they were cleaned and flow checked with 100k on 'em. just something to consider...

KAM

 

Interesting, Car has Accel 30# w/ maybe 500 miles on them.

Kevin I did check Dist angle and it was dead on assuming the balancer hasnt Slipped.

We will get to the O2 and to Running a compression test this friday and see what we get.

 

Interesting...there's another guy posting somewhere here that his brand-new Accel injectors dribble gas from some seam in the assembly when pressurized...

 

Ive heard Bad stuff about the Accel's before Too.

 

Are you certain there are no vacuum leaks? Like the IAC housing being loose? Check it, the BBK TB isnt machined exactly the same as the factory piece.

 

Yes, certain.

In Fact We even used a Tool which fills the Intake with smoke and none came out anywhere it wasnt supposed too.

 

How about the EGR valve or its related components? My brother's 85 TPI T/A and dad's 94 Suburban surged at idle when warmed up and both times were EGR related. On the T/A the EGR valve was bad. On the Suburban the EGR gasket was shot causing a vacuum leak.

 

If you disconnected the EST bypass, would it then go into open loop? If so, it does not necessarily mean an timing issue. It has been soo long working on a 3rd gen, i can't even remember if disconnectiong the EST will throw it into open loop.

Kevin

I would also do a snapshot if you can. Only real way to check all the sensor readings.

 

Dude I had the same exact problem that u have. My car would sit there and surge between 400-900 RPM's. It annoyed me so I checked everything under the sun to figure it out. You know what it turned out to be!! The spark plug wires! CHeck those and see if it helps. seriously give it a shot cause after I replaced those and some plugs for a tune up. All gone, not a bulb flickered after that!! oh and my tach bounced all over the place to. but after the wires it was smooth as glass at a constant RPM idel

hope that works!!!

 

I was thinking head gasket after "after fiddling." Only A few ways for steam to come out of the tail pipes, and you'll find all of them on the way to removing the heads.

As far as surging in closed loop.. I couldn't think of how a head gasket leak would only cause it then. Even something with the intake.. I'd think that it would be as bad or even worse when cold.

Hopefully the plugs will show you something.

 

We Will see, not sure when Im gonna get back to this car

 

Bing.... I remember RBob once saying that the $58 code has bug in the EGR section when used with a 8 cylinder. He has and I have on his tip, disabled the EGR routine in the bin. Kick up the enable temp to 255, and see if it quits surging. BW

 

Hmm, Ya don't say ?

The thought that it may be EGR crossed my mind when we first started diagnosing it, but since the Laptop showed 0% EGR duty cycle throughout the process, I sort of put it out of my mind.

Also, I am 95% Sure I disconnected the vaccum line from the EGR solenoid And it made no differance, but I will be sure to double check.

 

Is it using any coolant? Run it up to operating temp then shut it down for about 15 minutes. Pull the plugs on at time and look for any traces of water.

 

I had and still sort of have the same problem. I fixed the surging idle by replacing the IAC and adjusting it correctly. Also eventhough it NEVER set a code, the TPS voltage was a little low (45volts) I adjusted it to 54 volts at idle and now it only surges 50 to 100 rpm's which is way more acceptable than a 500 rpm surge.

I also have the water on the floor from the tail-pipes everytime I start the car, but this is when it's cold and I never see steam, it also goes away when it warms up. I think my situation is condensation from the long tube headers being hot and the huge y-pipe being cold. But I could be wrong.

You could also try adjusting up your target idle in your closed loop idle circuit table in the PROM.

Hope this helps!
Mike (1bad91Z)