RX7 Rough Idle Diagnosis Simplified
 

I have read numerous posts from FC and FD owners concerning rough idle or stalling problems and I have a suggestion that should make diagnosing this common problem a bit easier. Please keep in mind that what I am about to suggest is meant to be a quick STARTING POINT for rough idle diagnostics, as there are a number of different causes that can result in a poor idle. I have used this method with great success ever since I started repairing cars for a living in 1976.

First I like to start by trying to determine if the fuel mixture is too rich or too lean. I prefer to use the propane method for this, as it is an alternate fuel source that is controllable, predictable and safer than carb cleaner spray. I use a common propane bottle (torch kit) that I modified using common brass fittings so I can attach a 2 or 3 ft long vacuum hose to it (modifying the torch isn't required, using it as-is will work fine).

If the engine will run at all, have an assistant sit in the car and hold the engine at a constant speed in the 1500-2000 RPM range. (If you don't have an assistant, borrow a neighbor...if you don't have a propane torch, borrow his). Aim the UNLIT torch at the air filter (or intake duct if you are using the stock air box). Slowly open the valve to get the propane flowing into the engine. Pay close attention to how the engine responds. If the RPMs increase and the engine smooths out, your mixture is too lean. If the RPMs drop only slightly, the mixture was most likely in the normal range to begin with. If the RPMs drop considerably with noticeable roughness, a too-rich mixture is suspected. Another way to tell if the mix is too rich is by removing a large vacuum hose (the hose going to the brake booster is a good choice). You can control the size of the vacuum leak with your thumb on the end of the hose. If the engine runs it's smoothest with the air leak that you just created, it was running too rich. This is a generic way to determine fuel mixture. Obviously, a wideband 02 setup or a 5-gas analyzer would be the most accurate, but a good majority of us don't have that luxury.

If you determined that the mixture was too lean, now move your UNLIT torch from the air intake to any point in the engine bay where air might enter the engine, such as cracked vacuum hoses, injector O-rings, torn intake boots, etc. Pay particular attention to the intake manifold flange areas. This is where the vacuum hose adapter on the torch works very well. You can aim the propane exactly where you want it to help pinpoint a leak. It also allows you to keep the propane bottle upright so only gas is expelled and not gas/ liquid. I have found and repaired countless air leaks at the manifold flanges from the use of aftermarket gaskets which most often are made of a thinner material than OEM. Some people slather lots of silicone on both sides of the gaskets to help with this, and although it can work, I don't much care for oozing silicone into the port runners. I don't know of any silicone that doesn't break down from fuel vapors that exist after the engine is shut off.

This propane test is the second best way to find vacuum leaks IMHO. The absolute best way to find them is with a smoke machine that is used at some repair shops. A non-carcinogenic smoke is injected at low pressure throughout the entire intake tract. Wherever there is smoke, there's a vacuum leak. This is yet another luxury that most of us with FCs don't have, but you get the jist.

As with any diagnostic routine, don't rule out other causes of rough idle, such as:

Low/ uneven compression
Fouled spark plugs/ arcing plug wires
Incorrect spark timing
Fuel pressure/ volume/ pump voltage/ regulator internal leaks
Poor grounds/ harness plugs
Loose intake boots/ clamps

that is the longest "simple" post I have ever seen.

I actually thought it was good and informative post. I usually look for leaks with brake clean or carb spary, but propane is much better. Its also broken down for poeple with less car knowledge.

With rough idle problems as common as they are, and with so many people on here that are frustrated with it, I was hoping a Moderator would review my post and possibly archive it.......or better yet, make it a "sticky" so that everyone can use this simple method to track down vacuum leaks. It has worked very well for me for years. How 'bout it Mods???

I like this post :)

i hope it makes Archives.

The best way to hunt down vacuum leaks is to pressurize the intake with air out of the compressor with the engine off. That way you can hear the hissing.

James

Yep that's another way that I used to do it.....and while you have air in the intake tract you can spray a soap-and-water solution on everything and look for bubbles.

I did forget to mention that propane idea for idle afr is a really neat idea!

James

Thank you for the good words guys.....I'd love to have this at least Archived if not a "sticky".....it's so easy to do a propane check and costs hardly anything, with great results.

Yeah, thats a great idea. I'm gonna try that as soon as I get a chance.

good info, must be a sticky to cure all vac leaks

archive!

I pm'ed a Mod about it, with no reply. I would be happy to edit it however necessary, maybe condense it a bit if necessary to have it made a sticky. I use this method almost daily at work, and have repaired hundreds upon hundreds of cars successfully as a result. I am writing a similar article for the Global Vicinity site that might get more exposure.

well if they don't sticky or archive it than it's their loss, I already know what threat to look for. Thanks.

I'm gonna give this a try

Great thread.

great thread, lots of help only if you were here in nc i would bring my fc for you to fix it.

Yes I used this method to hunt down a ton of vacuum leaks on my car--leaky blockoff plates, bad gaskets, etc. Use this http://www.boostpro.net/prodtester.html with a reducer coupler if you are turbo. I think the stock turbo is 2 1/4" inlet. I used that tester with T04S turbo and used a 4" to 3" coupler. Spray some soapy water around the engine bay too and that helps even more. For example, I had a boost leak coming from my IAT sensor that I only noticed from spraying it with soapy water.

You can also buy the stuff at home depot/lowes. Even comes with hose clamps:)

Great info....simple in theory, assuming you have the tools... ;) +1 for sticky vote

I was thinking of re-writing this article (new thread) to include pics as well as the additional info given by others. I would give credit to those who have already offered additional good info. Then I could request archiving from the mods. What do you guys think?

Great post! I am going to try this later today if I get a chance...Thanks!

Does anyone have a documented guilde / shoping list of a good way to presurize the air intake? I'm sure I can manage to cobble something together, but why re-create the wheel?

Check for solid leaks at the PD too. It will lower your fuel pressure