What is this plug for? Chasing a rough idle problem.

[dsagers]

While changing spark plugs in my 1991 Q45 I thought I damaged the small plug on top of the large plug - See attached pic with the plug in the red outline box.

This is the plug that connects to the Ignitor(?) on the driver's side, top of the valve cover.

So I bought a replacement plug, but comparing the existing plug to the replacement, I don't see that I broke anything.

For starters, what is the small two prong plug for?

I'm chasing a rough idle problem that seems to go away by 2000 RPM, or else the engine speed is masking the problem. Also, I'm only getting some 9 to 10 MPG on the freeway.

[3Q Jay]

looks like the snap lock (i.e mechanical, not electrical) for the ignitor harness connector.

hows fuel pressure? leaky o-ring dumping fuel in?

[goody90q45]

Were any of the plugs you removed wet with fuel or did one or more look different than all the others (i.e. black)? If so you can trace that plug back to the problem cylinder(s).

[dsagers]

Appreciate the messages and help.

One plug was soaked in oil, so I replaced the valve cover gaskets. The car had sat for a while, so they would have dried out if they were wet.

Looks like I need to go to a power balance test and see which cylinder is missing and trace down the problem that way.

I also bought a can of BG injector cleaner and will try that too.

[Dabizzo1]

Having gone through the very same thing of a very rough idle with my 94 Q I've been restoring, I followed this path to finally cure the operation to a perfect idle.1)The entire airway (plenum removal and cleaning) including the EGR valve, passageways from the exhaust manifold(carbon removal), throttle body, IAC Valve, and MAF. Use new gaskets to reinstallo the IAC and throttle body.2) Checking EVERY SINGLE harness connector under the hood and applying dielectric grease on each and every connector.3) I changed all of the plugs with brand new OEM NGK plugs from the dealer...only NGK for the Q.4) My car is at 100k---knock sensors were dead, so both were replaced after throwing a check engine light.4) Making sure all coils are in working order for proper spark---they very rarely go out.

At this point, top motor breathing has been insured for the following step, which turned out to be EXACTLY what my problem was. Per the experienced words of Elwesso, Q45tech, Ceningolmo, Goody94Q45, GQJay, and Maxnix, Ohm test your injectors. They can be ohm tested with a multimeter at the harness, but this does not account for potential shorts in the harness itself, so I ohmed each injector from their tops, and after having 5 of my 8 injectors already brand new, 2 of the remaining 3 were toast. Ohm range should be within 10 to 14 ohms, and #3 was at 23.5 ohms and #7 was at 55ohms. Also, both of them had their diffuser tips completely rusted from ethanol damage(moisture in the gas lines---N. Texas car). Scour this site with the search function and you will definitely find answers that will help you. I owe my car's perfect operation to this site alone in terms of information. I hope I'm pointing you in the right direction. What makes this site so effective is a general love for these cars, and when combined with sequential thinking diagnosis, an effective solution is to be found for almost all failures. Good luck. Boris P.
Modified by Dabizzo1 at 12:46 AM 2/26/2007

[dsagers]

WOW!

Thanks for all the info and help. Looks like I have some work ahead of me.

[Dabizzo1]

I check this forum everyday, so feel free to ask me anything. I'll help in any way I can. Boris P

[Q45tech]

BE SURE TO AVOID ethanolized gasoline as the injectors, and fuel hoses and any part of fuel system where not designed to tolerate it.

Even today only 3% of US gasoline has it and in 1994 it was less than 1% of fuel produced. The system tolerated MTBE oxygenates sometimes but that has been replaced by ethanol........................

Running ethanolized fuel will just add expense in parts replacement.

Probably ok in an emergency but not as a daily fill!

Opinions vary on this but be warned!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[qship96]

BE SURE TO AVOID ethanolized gasoline as the injectors, and fuel hoses and any part of fuel system where not designed to tolerate it.

Even today only 3% of US gasoline has it and in 1994 it was less than 1% of fuel produced. The system tolerated MTBE oxygenates sometimes but that has been replaced by ethanol........................

Running ethanolized fuel will just add expense in parts replacement.

Probably ok in an emergency but not as a daily fill!

Opinions vary on this but be warned!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

guess all our cars in maryland will grind to a halt then?