Idle stumble I have tried everything Please Help!!

[SrS13]

I recently changed the head gasket on a 93 240sx.car ran fine for the first 40miles after the head gasket swap. However, recently after warming the car up and running it for about 20 minutes the car begins to stumble at idle and when accelerating under 2k. It is not an aggressive stumble but enough for concern and occurs sporadically.I have already assured that timing is correctly set and there are no ecu codes (other than 55). I have no mods.

I have cleaned the following parts thoroughly:- Throttle Body- Idle Air Control Valve- IAAC Valve- and the plugs that go to TPS, Distributor, Coolant Sensors, ETC.

I am not fiunding any fuel or vacuum leaks. I am becoming frustrated and cant solve this dilemma. Is there something I am overlooking?ANy help is greatly appreciated you guys have never failed me before and I only post as a last resort.

thanks

[r3v_v3ng3]

check if your exhaust system is clogged specially the converter

[SrS13]

the car has a new cat and a new oem muffler i doubt that there is a clog in the system

[r3v_v3ng3]

how's compression?

[motaro9424]

check you MAF sensor

[rioredstang]

Did you check your base timing and base idle? You have to unplug TPS to set timing. You need to take a volt meter and watch the 02 sensor to make sure its switching 200mv to 800mv. Sometimes a head gasket will coat the o2 and kill it.

[compactfean]

Volt test your Mafs. You should use black to ground and red to middle (white) wire. You should be sitting at 1v unless you are using a sohc la mafs then its about 1.5v. Tap the mafs with the back of a screw driver if the volts jump more than. 1 v than the solder in the sensor has broken contact is throwing f%$@ed signals. Simple fix. ..bust that bad boy open and fix the solder contacts. Also try a mafs cleaning.

[KennyV]

Make sure to retorque your head =)

[SrS13]

I Finally got some free time to check the compression on the engine and it wasn't pretty.

Cyl #1: 150

Cyl #2: 170

Cyl #3: 165

Cyl #4: 145

my numbers were a lot better before I changed the head gasket. I rechecked the torque on the head bolts and they seemed fine. What went wrong?

Thanks to all who have offered input I truly appreciate

[SrS13]

bump

[rioredstang]

Have you checked the plugs to see if it is running rich or lean? Could be a bad coolant temp sensor. Take a volt meter and check ohms across terminals. It could be out of range and not set a code. Check the plugs and let us know.

[SrS13]

both cooling sensor and temp switch were swapped out. I cant figure out what is causing the low compression.

[sx moneypit]

Pull the plugs and put a small amount of oil in the cylinders and see how the compression is.

[SrS13]

I add a bit of oil to each cylinder and compression improved significantly. If im correct this means the rings are bad?before I changed the head gasket compression was 170psi straight across and now its all over the place.

I need more input keep it coming. thanks all once again.

[SrS13]

something I forgot to add when the car begins to stumble the oil light turns on then turns off. Does that have anything to do with the low compression?

[sx moneypit]

That has nothing to do with the compression.If the car stumbles at idle and almost goes dead, that is why the oil light flickers.Does the car feel like it is dropping a cylinder or does it just fall on it's face?

[SrS13]

I dont know what it would feel like if it were dropping a cylinder but as the car is driven or on for long periods the stumble gets worse and worse to the point that the engine shakes at idle and looses power, i.e. it struggles to get up 2Krpms. I find it funny that it starts up fine and drives with no problems for the first few miles then suddenly starts to crap out.

[sx moneypit]

How is your fuel pressure?

[comatoseculture]

And what do your spark plugs look like?

[SrS13]

I will check fuel pressure later when i find a gauge but does that affect cylinder compression?

the spark plugs appear to have normal wear but have black deposit around them I hope you can see it in the pic

[sx moneypit]

Fuel pressure is not going to affect the compression.I am thinking that as the car is driven and the fuel pump heats up you are loosing fuel pressure.

[SrS13]

The car did not have that problem before I changed the head gasket. wouldnt the problem be because of the low compression?

[rioredstang]

Those plugs look lean. I would like to see o2 sensor, and coolant temp. voltage after the problem starts. You need to hook up a scanner or back probe pins at the ecu to see what is going on.

[SrS13]

I found a analog voltage meter and was able to check the voltage on the coolant temp sensor. I got a reading of 5volts coming from the plug of the sensor. Is that normal? I coulnt figure out how to get a voltage reading from the o2 sensor. What do you mean by probing the ecu?

Thanks again for all the help

[sx moneypit]

Before you do any testing on any computer controlled circuits you need a high impedance digital meter to keep from screwing something up.Download a copy of the factory service manual,it will show you how to back probe your ECU.

[rioredstang]

You can back probe the o2 sensor to get the reading. Back probing means you don't unplug the connector. You can carefully slide a paperclip behind the wire in the connector. We want to see what the voltage is that the computer is seeing while the circuit is loaded. Now ground the black (-) lead to battery negative. Touch the red (+) lead to the paperclip while the engine is running. The o2 sensor should read switch back and forth from 100mv to 850mv. Now rev the engine to 2000rpm and watch the volt meter. Does it switch or does it fall to about 100 to 200mv? If so the motor is lean.

I think you may have a fuel pump beginning to fail.

[SrS13]

i back probed the 02 sensor and the coolant temp sensor per rioredstang's instructions. These were my results using a digital multimeter:

Upon starting the engine the cts read 1.45 volts; the o2 sensor would fluctuate between 550-850 mV.

as the engine warmed up and the stumble began the CTS read 1.06-1.1 Volts; the 02 sensor began to fluctuate between 295-850mV

as the stumble got worst and i reved it under 2k the CTS remained at 1.06-1.1Volts and the o2 sensor would drop from 850Mv to 100mV.

[sx moneypit]

Did you ever check the fuel pressure?

[SrS13]

I checked fuel pressure today at idle i get a constant 36psi even during the stumble.

when the stumble gets worse and I loose power from the engine (struggles to travel at 10mph) and the engine struggles to stay on the fuel pressure gauge reads from 43psi and it jumps to almost 50 when the engine is about to stall.

The O2 sensor during that same period where the engine is struggling to stay on read under 100mv.

if I turn it on later today or tomorrow it will start up fine and then the same problem will occur.

[sx moneypit]

Did you ever check the mass airflow sensor?