Dropping revs between 2-3k

[kyngfish]

Hi All,

I checked previous threads and I think I have some idea of what be happening, I just wanted to see if you all could help tell me what to troubleshoot first.

This weekend I replaced the spark plugs, cap and rotors, Monday the car was running fine, but to be fair, I wasn't really working it very hard.

Last night I installed the Steve Wong chip and ran into several things:

• The car is idling solid, started right up, nice power delivery in the lower revs, everything the SW chip advertised.
• The car randomly revved 1 time - this is no longer happening
• At high RPM, so 5000 and above, I was getting a weird rattling sound, kind of near the tailpipe? Cat? This also has disappeared (driving aggressively this morning)
• Revs randomly dropping under load between 2-4k - this also went away for now

Figuring it might be an ignition problem, I checked the wire connections at the distributor cap, and at the spark plugs, and all looks good, though if I keep getting the problems this weekend I will pull all the plugs and reinstall.

Could this be an issue I created when I was fiddling with the DME installing the chip? Fuel pump? Filter?

Any other pointers?

Thanks,
Martin

[Tippy]

Revs dropping are usually speed and reference sensor related.

[kyngfish]

You may be right, but aren't those just coils of wire? Seems odd all that would happen when really just worked over ignition related parts over the weekend...

[Quicksilver]

"Things work right up to the point where they fail"
The issue might not be related to any systems that were worked on recently.

What do you mean by "RPMs dropping"? The RPMs only drop after power falls off or completely cuts out. Try to explain EXACTLY what you are seeing happening. It can't be happening 100% of the time otherwise you couldn't exceed that RPM and you would describe it as a rev limit.

[kyngfish]

OK, fair enough. I'll be ON the throttle, accelerating, and all of a sudden it will drop revs from 3000 to 2500, then come right back up, almost like it had gas, or spark, and then it didn't for a split second, then caught itself again. It only happened twice so I'm not certain it is only between 2-3k, but idle is rock solid, and it appears to be happening when I'm applying pressure.

During this entire exercise my foot will still be on the throttle, so I haven't let up at all.

[Tippy]

Originally Posted by kyngfish

OK, fair enough. I'll be ON the throttle, accelerating, and all of a sudden it will drop revs from 3000 to 2500, then come right back up, almost like it had gas, or spark, and then it didn't for a split second, then caught itself again. It only happened twice so I'm not certain it is only between 2-3k, but idle is rock solid, and it appears to be happening when I'm applying pressure.

During this entire exercise my foot will still be on the throttle, so I haven't let up at all.

I spent 6 months with that same issue before throwing all the Motronic stuff on the ground and wiring in MegaSquirt.

Never figured out.

[kyngfish]

So what you're saying is it's probably a DME issue? I guess I can try to put the stock chip back and see what happens.

[A horse with no name]

Originally Posted by kyngfish

OK, fair enough. I'll be ON the throttle, accelerating, and all of a sudden it will drop revs from 3000 to 2500, then come right back up, almost like it had gas, or spark, and then it didn't for a split second, then caught itself again. It only happened twice so I'm not certain it is only between 2-3k, but idle is rock solid, and it appears to be happening when I'm applying pressure.

During this entire exercise my foot will still be on the throttle, so I haven't let up at all.

Possibly poor fuel pressure. Your fuel pump may be on the way out. A simple fuel pressure test 'may' tell you. If it hasn't happened again, it also may just be the ECU resetting itself with Steve's Chip... Why not drop him a line and ask him what he thinks?

[kyngfish]

Actually it got worse on the commute home, it happened at almost exactly the same place, making me think it has to do with the temperature on a component.

I did talk to Steve, he thinks it's the ECU or Relay. Ordered a relay, hopefully that's it.

[A horse with no name]

BTW, when this happens, is it a ' bucking' motion or more of a stall?

Being that the engine starts, it appears that your reference sensor is OK. If it is a sensor, it would likely be the head (heat) sensor and or the speed sensor.

While you are waiting for your new relay; do you have a multimeter so as to check the head (heat) sensor? It's the top white one of 3 on the bracket on the left side of the engine. The second one -down- is the speed sensor, with the reference sensor being on the bottom.

To get at it, just take the plastic heater intake pipe to the heaters blower motor off.

This is an excellent 'how to check' for your present DME, including how to do an 'emergency' repair...http://clarks-garage.com/shop-manual/fuel-05.htm

Also have a read of this Pelicans post for the head (heat) sensor... engine cuts out.

[DRACO A5OG]

OP, lets back track your steps.

Can you post a pic of your DME on the harness. Did you feel a "CLICK" when you put the DME back on?

Let's remove the DME (ECU not the relay) and take a look at your connectors on both the DME and Harness.

I ask this because sometimes we tend to not fully lock the DME onto the harness or accidentally bend the connectors or there could be corrosion we missed.

If you did not feel a click, the connectors are not fully seated.

A sudden drop in RPM's sounds like loss of signal to the ECU. I hope this makes sense???

"rattling" this is very subjective. So please be more descriptive, was the engine warm to hot when this occurred or when cold? Did it sound like a metal trash with a stick inside the can sound or a more of a knocking/pinging sound?

Always a good idea for us 3.2's to have a spare DME RELAY or a 3.2 buddy close by to lend you one in an 911.

[kyngfish]

Hi All, thanks for the tips.

- Yesterday it got very bad, basically any time I put the car under load, from 1k RPM to 5K RPM it was jerking like a bad shift.

- The strange thing is it only happens at a very specific temp, around 2mm below the 8 o'clock hashmark - once my car got up over the hashmark for a little bit, the problem went away completely.

- I am picking up a DME relay today on the off chance that is it, but I also think it's the ECU.

- Car starts and idles fine. Runs strong otherwise.

DRACO: When I replace the relay, I'm also going to check the seating and the ECU, but I specifically remember checking for the click. The only thing I didn't do, is fold the tabs back down, and the cover for the ECU is sort of loose on top. The rattling went away, but it sounded like a stick in a metal trash can.

HORSE WITH NO NAME: It is definitely a bucking motion. Like I'm just starting to learn how to use a stick shift. Unfortunately, I do not have a multimeter.

[DRACO A5OG]

Take the time to retrace your steps on the new chip install and proper re-assembly of the ECU. I know it is scary because you want to be gentle but the DME is pretty robust and can handle some man handling. Everytime I watch Steve work on the DME's, I cringe as he laughs at me so they can take a little roughness.

If you can post pics of the halves and how the chip sits in the chip socket, halves assembled and DME in the harness would help. Heck, post the connectors too.

Don't worry about the tabs, I use packing tape to keep the cover from rattling as my tabs have broken off. You did place the white film shield in the right place, correct?

Metal Trash Can Rattle? Okay that is most likely coming from the chain tensioners. If this happens again. Use a stethescope of screw driver and place you ear on the chain tensioner boxes to verify the chains are rattling.

[A horse with no name]

Re the 'bucking motion'... I guess that it will be best to first see how you go with your new DME.

Just an additional thought on the problem. It is a very simple and easy thing to check.

I had read a thread where a similar problem was fixed by simply tightening (retorquing) the intake manifold bolts. Another Pelican had made the suggestion, and in this case, it was 'the fix'...The bolts were loose.

For a quick reference: The torque value is 18 ft lb, 8mm hex

Re not owning a multimeter....I would though consider picking up a multimeter. It doesn't have to be an expensive one, such as a Fluke. Without the help of one, it is near impossible to check for some of the most very basic electrical issues that you may have with your 911. Our cars are noted for having grounding issues that can affect, a most common issue, your door or hood light function.

E.g. Your lights don't go on anymore when you open the door. Is it a worn out switch? A ground; loss of power? With a multimeter you can find the issue. [I]BTW, I don't sell them

Draco, including numerous others Pelicans, have lots of experience on these matters so don't 'you' worry tooo much about it, as help is only a 'click' away.

.

[Quicksilver]

Another diagnostic point is when it cuts out see if the tach drops.

-----------------------------------------
A little "all over the map" comments...
For bucking "take your head off - on/off/on/off/..." check if the injector connector in front of the engine is loose. I would also check all the ground attachment points in the engine compartment.

You are describing a very sharp cut out? For the fuel pump I would expect it to take a second to completely cut out. A little soft. Not so jarring. And the "come back on" would be soft too.
I would want to know if the ignition and/or the injectors was cutting out. For the ignition you can tape an inductive timing light to the rear window and drive around to see what it does. (plus you can give anyone watching some great acid flashbacks.)
For the injectors I would make a "Noid light" with a long lead so I could see that also. I suspect that someone here like Sal could come up with a simple noid light circuit to run in parallel with the injector for this purpose.


You did mention that it happened at a specific temperature. (I would be very interested if that continues to be the case.) That makes me suspicious of the cylinder head temp sensor but I don't know how you could test for that. The other thing that comes to mind is that the thermal expansion could be causing something to move "just so" so you see a failure at that temp.
(I don't think the AFM temp sensor could do that because it measures the incoming air and that doesn't heat up.)

[A horse with no name]

Re quote: "but I don't know how you could test for that."

He would need a multimeter to be able to check the head sensor. It's very easy to do. As we know, it's the top white one, of the gang of 3 sensors, that is located behind heater blower motor.

When cold it should be around 2500-3500 ohms, when hot around 200-300 ohms. When it's bad usually it causes the car to run too rich and makes the car hard to start.

[kyngfish]

Can anyone show me where the injector connector is? I did the spark plugs cap and rotor this weekend, maybe I jogged something? Also does someone have a picture of where the head temperature sensor is?

[A horse with no name]

Originally Posted by kyngfish

Can anyone show me where the injector connector is? I did the spark plugs cap and rotor this weekend, maybe I jogged something? Also does someone have a picture of where the head temperature sensor is?

The head temp sensor is bolted into the rear of the left head. It has a white connector, including the wire harness itself. As mentioned, the plug into the ECU, is on a vertical metal bracket, and is the top white one, of the gang of three, located right behind your heater blower motor.

Where you would connect a fuel pressure gauge is located on the fuel rail rack which you will clearly see on the left side of your engine. It is behind your distributor,

[A horse with no name]

If you were to follow the white temp sensor wire to the sensor itself, this is how you would get at it.

If you were to point a flashlight up into your left wheel well, you will see the white head sensor wire coming out from the under the hood connection, through tin, to the rubber plug to the head. The speed and reference sensors are also located in this area. You can see the two of them.

[DRACO A5OG]

Originally Posted by kyngfish

Can anyone show me where the injector connector is? I did the spark plugs cap and rotor this weekend, maybe I jogged something? Also does someone have a picture of where the head temperature sensor is?

Black rectangular four pinned connector behind the heater blower motor on chassis cross bar, stick your head back there, can't miss it