Rough Idle 4.0 SOHC

[pingj]

Hi All,

I'm new to this forum, but I have been out on others for many years.

I just finished doing the timing chains and tensioners on my '98 SOHC with 199,550 miles on it. I wish I could afford the $ to do a complete rebuild but times being what they are I had to get off as cheap as I could.

Anyway, the engine ran ok before pulling it to do the timing chains. Put it all back together and put it back in and after a few mistakes, PLUG Wires out of sequence and a EGR vacuum leak the beast is back.

Except now it is harder starting. Not really hard but before it caught on the first crank. Now it takes two short cranks.

Also after it catches the idle will bounce around, going from ~ 500 to 1200, then back down to ~ 500.

Tapping the trottle will increase RPM but until the engine has run for @ 3 minutes the RPM keeps jumping. Once it is warm if you shut it off and start it back up it runs up to ~ 1200, then drops to about 600 which is where it idled normally before I pulled it to do the timing chains.

I just replaced the IAC before the timing chains.
I found indication of a vacuum leak on the upper intake gasket.

I cleaned the intake and throttle body before putting the engine back in.

I did not replace the lower intake gasket, but after doing some reading here I wish I had. Money was tight so I tried to cut corners as much as possible.

Sorry for the long explanation but I'm hoping you can help with any ideas.

I'm going to clean the MAF and TPS sensors this weekend, and my buddy has a new Crank Sensor I'm going to put on. Not that I think that is the problem but what the heck, its free.

Looking forward to hearing your suggestions.

 

[llamaking122]

If i had to bet it would be on the intake manifold gasket

 

[2000StreetRod]

cold start high idle

Typical symptoms for a leaking intake manifold gasket (upper or lower) is high idle speed at cold engine start. As the engine warms the old dried out gaskets expand and seal better reducing unmetered air intake. I had leaks in the upper and lower intake gaskets. The airflow left marks that were visible. See Starting my 00M12 Installation

Do you have access to a scanner to read the engine coolant temperature? The ECT has a significant impact on the air/fuel ratio. Maybe there's a bad connection causing the ECT to fluctuate. If you don't have access to a scanner you can try reading the resistance with an ohmmeter. Since access to the ECT sensor is limited, you can disconnect the PCM connector and measure the resistance between the light green/red wire and the gray/red wire. BrooklynBay has the ECT table posted: ECT Chart

Also, make sure there is a little slack in the throttle body cable.

Was the idle speed erratic after you changed the IAC valve before replacing the timing chain components? Some members have omitted installing the figure 8 gasket which results in an intake leak. Some aftermarket IAC valves have a very thin gasket that doesn't seal well.

 

[pingj]

Do you have access to a scanner to read the engine coolant temperature?

I do have a basic scanner but not one that will allow me to read the temp. Its just the basic of scanners.

Also, make sure there is a little slack in the throttle body cable.
There is slack in the cable.

Was the idle speed erratic after you changed the IAC valve before replacing the timing chain components? Some members have omitted installing the figure 8 gasket which results in an intake leak. Some aftermarket IAC valves have a very thin gasket that doesn't seal well.

After replacing the IAC for about 2 weeks the idle was rock steady, then one day I got a "low vacuum bank 1" code and the rough idle came back. After doing the timing components, once the engine is warm it idles great, truefully better than it ever has.