Stumbling Engine During First 1/2 Mile of Driving

[thom4782]

Hi all:

I've noticed what seems like a small, but nonetheless, irritating problem. When I start the car and drive the first half mile, the turbo engine seems to stumble when its accelerates. After a mile or so, the issue completely disappears.

Looking for some ideas on what this might be before the car goes into the shop a couple of days from now to have the idle control value replaces. When the ICV gets hot, it allows the rpms to stay at about 1400 when idling. A brief tap on the valve and the rpms return to 750.

Best

Thom

[v2rocket_aka944]

unplug the ECU coolant sensor (blue plastic connector) and see what happens.

[Jfrahm]

Get it stumbling good, maybe driving towards your house, and cut the engine and put the clutch in. Roll to a stop and check the plugs right away to see if one is wet, if all of them are wet, or what.

[951and944S]

Originally Posted by Jfrahm

Get it stumbling good, maybe driving towards your house, and cut the engine and put the clutch in. Roll to a stop and check the plugs right away to see if one is wet, if all of them are wet, or what.

Yep...., sounds like plug wires.

Damp/foggy conditions increase ability for one to short to ground.

T

[Tom944]

Could be the O2 sensor heater. If it fails the engine will run badly until the sensor warms up from engine heat. Will start fine since O2 sensor is not used for cold start and first few seconds.

[acar24]

Probably running too lean, = vac leaks, fuel pressure, injector. vac hose update under intake good idea when replacing isc. spark plugs carbon coated or too wide gap. good luck.

[thom4782]

Replaced ICV and its hoses as well as the heated O2 sensor. Engine runs smoother at all RPM ranges, but still there is a slight stumble during first acceleration. Next step is to inspect spark plugs.

[DannoXYZ]

There may be only one issue here, and it may not even be defective part. But you're more than welcome to replace tonnes of perfectly-working parts with brand-new perfectly-working parts.

Measure resistance of ECT at two wide temp-range, freezing and boiling water. Then compare with graph in FSM.

Was this stumbling present during summer?

[thom4782]

Replacing the ICV fixed an intermittent high idle problem...the valve was sticking after the car warmed up. Replaced the O2 sensor b/c it was close to the typical replacement mileage. Looking into the spark plugs because I have never inspected them and want to see if they suggest anything going on.

It's possible that the problem occurred during the summer. If it didn't what would that tell you?

[DannoXYZ]

Just trying to see if it's temperature-related issue.

Currently, does car do cold-start high-idle? After you crank and it starts, does it hum at about 1200-1300 for a minute or two?

[thom4782]

Ok, this is embarrassing, but in the spirit of full disclosure, here goes.

I've been chasing two problems.

Problem 1: High Idle. This problem occurred after the car was fully warmed up. For example, typically, I would drive for 20 to 30 minutes. All was well. Then I'd shut the car off. After restarting within 10 minutes, the car would idle between 1400 and 1800 rpms. If I tapped on the ICV, the idle would return to normal. For this reason, I replaced the OCV and its hoses since these parts were original.

Problem 2: Hesitation in the first 1/2 mile of driving. This is the embarrassing part. I would start the car. It would idle at 800 rpms steady. After a few moments, I'd drive down the long driveway at about 20 mph. When reaching the street, I'd accelerate and the car would seem to 'miss' one or twice in the next 1/4 mile. After then, the car would run as smooth as can be. Well, its looking like I was feeling a slight dip in the road and not an engine miss. After starting today and driving around where things ran smoothly, I returned to the road where I'd felt the issue. I drove down it three time and felt the hesitation each time. Well if the engine can't tell what road its on, the the issue must be the road itself.

So, one problem was parts related and the other was not.

[DannoXYZ]

Hahahah!!! Whew, glad that's taken care of!

Seems your cold-starts should have higher RPMs though...

[thom4782]

Hi all:

Well, I thought this was fixed, but apparently it's not. So, I'm looking for some suggestions on next diagnostic steps. First, let me summarize the problem and steps I've taken so far.

Problem

1. Engine cold (typically overnight)
2. Starts immediately and idles steadily at 850 RPMs
3. After a moment or two, runs smoothly as RPMs spin up from 850 to 4000 RPMs with the clutch depressed (not under load)
4. Drive first 1/4 mile with the engine cold; engine runs smoothly under light loading.
5. Next 1//4 mile with engine still cold, engine stumbles intermittently (2 or 3 slight hesitations) as the engine spins up under heavier load.
6. After first mile or so, engine runs smoothly without stumbling regardless of loading

Work Done so Far

1. Replaced Idle Control Valve
2. Fixed intake gasket leak
3. Adjusted fuel trim
4. Replaced spark plug wires
5. Verified Throttle Position Switch functions correctly

My best guess as to possible causes

1. Vacuum leak (although none seems to be found after several checks)
2. Fuel filter
3. Fuel pump
4. Fuel pressure regulator
5. Fuel pressure damper
6. Fouled spark plug, even though they were replaced 2000 miles ago
7. Fuel injector

The issue that has me baffled is why does this only happens when the engine is cold. Any advice would be greatly appreciated

[kdjones2000]

If you are trying to push the car while the engine is still cold, you can almost expect some hesitations. Mine does that as well.

[DannoXYZ]

Originally Posted by thom4782

Work Done so Far

1. Adjusted fuel trim
2. Verified Throttle Position Switch functions correctly

How was this done? Please tell steps involved in each. thx.

[DannoXYZ]

Originally Posted by thom4782

Hi all:

Well, I thought this was fixed, but apparently it's not. So, I'm looking for some suggestions on next diagnostic steps. First, let me summarize the problem and steps I've taken so far.

Problem

1. Engine cold (typically overnight)
2. Starts immediately and idles steadily at 850 RPMs

Originally Posted by kdjones2000

If you are trying to push the car while the engine is still cold, you can almost expect some hesitations. Mine does that as well.

My 951 can be pushed to 25psi within 5-seconds of starting with no stumbling. I live in same area as OP. Trick is to measure all parts and ensure they are within factory spec:

- ECT sensor's resistance
- IAT sensor's resistance
- intake-manifold vacuum with cold engine (actual number of in.Hg or kpa)
- intake-manifold vacuum with warmed-up engine
- vacuum at FPR-nipple with warmed-up engine (use vacuum-T to keep FPR functional)
- vacuum at fuel-dampener with warmed-up engine
- measure fuel-pump flow (30-seconds)
- measure fuel-pressure in rail at idle
- measure fuel-injector's individual flows (15-seconds)
- measure TPS's position output -voltage at idle
- measure TPS's idle-switch state at idle (closed throttle)


These measurements in numbers will pinpoint exact problem when compared with numbers in Workshop Manual. I suspect one of these three will be out of spec when measured:

- IAT resistance
- ECT resistance
- TPS idle-switch state (zero or infinite-ohms)

Why? Because car starts and idles at 850rpms. With 30-40 degree temps we've been seeing, DME must engage cold-start enrichment-cycle with extra fuel and high-RPM idle of 1200rpms for at least 30-seconds immediately after starting. And if it still detects cold-engine, it will continue with fuel-enriched high-RPM idle; sometimes for 5-minutes in really cold whether.

So... I suspect those three items are most likely culprits, maybe just one. First, IAT & ECT sensors signals to DME that it's COLD out and it should use cold-start enrichment. If those sensors are bad (or wiring to DME is bad), then DME never detects it's cold outside and never engages cold-start enrichment cycle.

Second, DME only uses ISV to adjust idle-speed when it detects throttle is closed. Even if it clicks, there are many TPSs that have electronically tested to be bad and not signal closed-throttle position correctly. Even better to test TPS idle-switch at DME connector to verify wiring in between. So even IF DME detects COLD weather outside, if it doesn't see that throttle is fully-closed (idle-switch), it won't activate cold-start enrichment cycle and high-idle speed via ISV.

[Bukowski]

Thanks for this discussion

I just noticed today similar high RPMs in the warm up phase. I also want to say the RPMs (for my NA) were unusually high - near 4000 - while near 20-30 mph.

Also when my RPMs dipped a bit, I can notice a shudder/struggle.

Ok thanks for letting me insinuate some of my own problems in this thread.