97 NA850 troubleshooting, no codes, out of ideas

Post by **div4scpro** » 02 Aug 2011, 15:49

97 NA850, automatic, late year model with Motronic 4.4, 191K miles
Symptoms: no codes at all
engine cold - runs fine
engine after 15-20 minutes - light throttle surging or stumbling that gets progressively worse, doesn't begin as
soon as engine is up to operating temperature
starts fine hot or cold
idles like sewing machine at 840 hot or cold
heavier throttle application - stumbling smooths out but seems? like less power than when engine cold
Details: had PO300, 303, 305 earlier - replaced all ignition, bougicord wires, NGK V-power plugs, Bosch cap & rotor, and coil
plugs all show white/light tan color
injectors cleaned and flow tested, all new injector seals
fuel pressure builds to 43 immediately, hours to bleed down
cylinder head rebuilt and new valve stem seals
compression test - 190-205 on all cylinders
crankcase ventilation all replaced - tested and no pressure at oil cap, vacuum as expected
new timing belt and tensioner - timing is exactly on the mark
coolant temp sensor - OEM new 15 months ago and new thermostat
new purge valve and inspected vacuum all lines and elbows to cannister and throughout engine compartment
IAC valve cleaned
new Bosch MAF sensor
fuel pump relay, no symptoms but rebuilt IAW Ozark Lee's post

Questions:

I don't think I have left out any relevant data, but just ask
without codes I don't know what else to check
any ideas greatly appreciated

Andy

Post by **kcodyjr** » 02 Aug 2011, 16:05

Sounds like you have a vacuum leak. The delay would be explained by the time it takes for rubber hoses to warm up and soften.

This could be your air intake hose from the airbox to the throttle body, or it could be a vacuum hose just about anywhere.

If nothing jumps right out at you, start spraying carb cleaner around with the engine running. When the engine surges, you've found the leak.

Post by **div4scpro** » 02 Aug 2011, 16:26

unfortunately have done the leak/spraying test, cannot find a leak anywhere, have sprayed every vacuum connection I can find

Post by **kcodyjr** » 02 Aug 2011, 16:34

I recall there's a vacuum line under the battery tray, for cruise control, did you check that?

Also, did you squirt around the base of the vacuum tree, and where the TB meets the manifold, and where the manifold meets the cylinder head? Air leaks can be just about anywhere.

This really sounds like it goes lean when you step on it - so either it's sucking air, or it's mismeasuring. I suppose a clogged fuel filter could do it too, but it sounds like you've been through fuel delivery pretty good.

As a test, you could try disconnecting (and capping) all of the vacuum tree ports, and see if it behaves any better with all vacuum accessories disconnected. That leaves you with an air path smaller than a shoe box.

Post by **Ozark Lee** » 02 Aug 2011, 18:30

Double check the coil wire connection at the distributor cap - we have had a rash of failures at that point and it seems have similar symptoms. I went through it a couple of weeks ago and the car actually died on me on the way home from the cabin.

I have bought so many of these cars that I can't remember what I have done on all of them (in my mind they are all through stage zero). The plug wire date code said that they were original to the car as was the cap and rotor. A new set of plugs, new wires, and a new cap and rotor and it hasn't had a hiccup since.

...Lee

Post by **div4scpro** » 02 Aug 2011, 19:37

will do more vacuum checking under the battery tray. frustrating but i can't find anywhere that says what the ideal vacuum would be under different operating conditions. will also try to hook up vacuum gauge to cockpit so that I can evaluate under road conditions. will also try to see if i can hook up fuel pressure gauge inside to watch under road conditions also. did already change the fuel filter. THANKS
Andy

Post by **kcodyjr** » 02 Aug 2011, 22:20

div4scpro wrote: engine cold - runs fine
engine after 15-20 minutes - light throttle surging or stumbling that gets progressively worse, doesn't begin as
soon as engine is up to operating temperature
starts fine hot or cold
idles like sewing machine at 840 hot or cold
heavier throttle application - stumbling smooths out but seems? like less power than when engine cold

If you warm up the engine, observe the problem, shut it off, give it a five-count, then start it back up, and immediately put it in gear and take off briskly, does it respond as it should, as it did when it was actually cold?

If so, it's got nothing to do with whether the engine was hot or cold, but whether the ECU was running open or closed loop. That doesn't quite exactly coincide with reaching operating temperature - someone else might know the criteria exactly?

If that's the case, I'd start suspecting your front O2 sensor, though it's strange it hasn't thrown a code if that's what it is.

I think we can rule out ignition; if a weak spark was in the picture, combustion would be incomplete, and the plugs should be fouled, plus it should be offending your nose enough to have merited a mention.

Post by **jreed** » 03 Aug 2011, 05:22

One area you didn't mention checking is the throttle plate and body for oil build up or damage. I found quite a bit of oily residue around the throttle plate on my '97 855 GLT (at ~140k miles). When I cleaned that up it seemed to have improved the idle stability.

Post by **div4scpro** » 03 Aug 2011, 07:57

okay more info:
throttle body and plate area are clean
have a new bosch front O2 sensor installed, shows open loop and closed loop on the scan gauge when it is supposed to
sprayed the whole intake area, intake hose, throttle body and no surges or changes, either in idle speed or vacuum reading
checked and sprayed hoses under battery tray and evap canister, no change
have disconnected everything from the vacuum tree, makes no change in symptoms or vacuum readings
I disconnect the battery and discharge the residual charge to reset fuel trim after every troubleshoot
checked all ignition connections, good

could the throttle position sensor have anything to do with this? i read that it has a significant impact on acceleration which is when the symptoms are worst. is there any way to test this device, resistance, etc?

Post by **erikv11** » 03 Aug 2011, 07:58

People drive for years with a leaky elbow on the cruise control, under the battery tray, even on turbo cars, so I doubt that but check the lines sure. Also vacuum lekas are overall much less of a problem on the NA cars.

The symptoms you describe are similar to when I ruined the TPS on my car. Got solvent inside it while cleaning the throttle body. Car would stumble only at high revs, and never threw a code. In your position I would grab one from a junkyard and try it out. I have read there are ways to test the resistance of the TPS but for one with intermittent problems not sure what that would get you.

Or send me a PM, I can send you a one to test it with. I have a spare from a recent engine swap, from the junkyard engine. Has not been tested but in all likelihood it is good.