Rough, shake when warm

[gkdada]

My 92 Q (130000 miles) is smooth as silk when cold (the first mile or two in the morning) but becomes a little rough and sluggish after that. Also there is a mild shake at the stoplight that's not there when cold. No CEL and no excessive smoke.Just did a tune-up with all new PFR5G-11 spark plugs. Didn't help, in fact seems to have gone a little rougher. Can I do anything else?

[Q45tech]

Power balance test to find the bad injectors.

[elwesso]

welcome to NICO.

Check out http://www.q45.org for tutorials on checking injectors.

[gkdada]

Thanks, elwesso and Q45tech. I wish I had a similar forum where my computer expertise is of help to others. You are really great.

Some more pointers (and things I discovered in past 2 days):

1. The sluggishness that was there before I changed the spark plugs is definitely gone.

2. The shake is only at idle and very low speeds, like 5 mph. Above this speed, car has a hint of vibration (which is what I meant by rough) but only till about 50 mph or so. At freeway speeds, it is really really smooth. The moment you get off the freeway, vibration returns.

3. There was an occasional rattling sound before which has now turned to a constant rattle, but only heard when idling (in any gear position). I was told I need a muffler (cat) job. Could this be the cause the shake and vibration?

Thanks.

[maxnix]

Try some BG Quick Clean in your engine as it's idling waiting to go into the bay for the oil change, which must be done immediately after shutting it off.

Without a maintenance history, especially concerning the intake and fuel system, we are just shooting in the dark.

If vibration is about 50 mph in 3rd and 70 at lockup (OEM diameter tires), it is the third motor (transmission) mount that needs replacing. Exhaust hangers are done at the same time.

[gkdada]

Vibration is at low speeds all the way upto about 50mph (expressways here in bay area). At freeway speeds (65-70), the vibration is COMPLETELY gone.

I haven't tested for the exact speed at which vibration stops OR the smooth/suddenness of the changeover.

Thanks.

[maxnix]

Still no maintenance history.

With CA crapgas, I wouild run a bottle of ISO-HEET every other tank and BG44K once a month.

Have you ohm tested your injectors?

[gkdada]

Sorry.

I bought this car two years back at 121xxx. No maintenance records available upto that point and all I can figure (from carfax) was that it has been in very little use since about 2001. The maintenance log entries run out at about 60000 miles. There are no signs of major engine work.

Since then I have put about 9000+ miles. Oil changes every 3000 miles (or less). Transmission oil is clean and fresh.

The car was running like a dream for the first 7000 miles, but the shake has been going on for the last 2000 miles. The mileage has always been 13mpg city and 15-16 on highway, which I think is a little on the lower side. CEL never went ON.

Other than oil changes, I have not touched the engine at all (till the tune up this week). The spark plugs pulled during tune-up (IIRC) were autolite and most of them were somewhat coated with oil and a couple had their ceramic body browned. Replaced with PFR5G-11s.

Thanks.

[maxnix]

OK, you need to consider pulling the plenum, replacing all the hoses and the valve cover gaskets, the KS and their harness. Get your parts from Joe.

You will pull the whole rail injector out and ohm test the injectors directly. Either replace from Joe and/or send to Deatschwerks if you want to bench clean them.

Many threads on how to do, plus the FSM. Most threads are in IOM.

You mileage infers bad injectors. Clogged or failed is difficult to tell.

Before you start, you may want to consider wiggling the MAF connector to check for contact. definitely inspect it and the wiring when it is off.

I presume the timing chain guides have been replaced with new metal backed ones? Tensioners? Oil pump chain?

[gkdada]

Thanks.

I will get on those one by one. From the easiest to more challenging ones. MAF and injector ohm check first, I guess. :-)

[maxnix]

Thanks.

I will get on those one by one. From the easiest to more challenging ones. MAF and injector ohm check first, I guess. :-)

You got it. The good thing is that if you remove the plenum, you can ohm the injectors directly instead of through the harness. An injector can ohm within specification, but still be dirty and not operate properly, so the test is just a beginning, not a definitive end.

[gkdada]

The result of ohm test:Pass: 1 - 12.6, 7-12.3Fail : 2 - 34.3, 3 - 390(!), 4 - 54.5, 5 - 26.8, 6 - 54.4, 8 - 16.9

Now, it is not possible that SIX of the eight injectors are bad, right?

Here's the twist in the tale:

For entirely circumstantial reasons, my boss drove the car for about 40 miles (and I was there). He drove hard with some sudden accelerating and some high revving. At the end of those 40 miles, the problem was 90% cured.

By next day (yesterday), you can only feel the shake if you are looking for it and the vibration at low speeds is COMPLETELY gone!

That's a definite cure for shakes and if anybody is interested we are in San Jose area.

[Q45tech]

An injector is just a coil, an electromagnet with the core attached to the pintle valve.

Changes in coil resistance will change the strength of the magnetic field pulling open the pintle.

The injector will take longer to open and flow less fuel during the cycle, so the cylinder runs leaner than the normal injector feed cyclinders and may will misfire or weakly fire and partially burn some of gasoline.

Corrosion inside injector may change as the injector warms up from underhood temps and just running. The injectors must be tested cold and hot to note the changes.

25 year old technology should not be a surprise or new to a car owner.