bucking 87 carrera

[ssscarrera87]

I have an 87 carrera coupe with 120,000 miles I have just done a engine rebuild.New pistons,rings,liners,a valve job, new cam shafts and a new clutch while I was at it. I am about 600 miles into the break in period and something new has popped up . Between 2800-3000 rpm the car starts bucking,I am not hard on the throttle just driving along,this will happen in any gear,when you accelarate it goes away.
I have checked the coil, not it , i have tryed the 02 sensor does not change,the throttle position switch,I have even swapped out the airflow meter,and nothing has worked.
Anybody have some good ideas,because right now I am scratching my head.
Thanks
Sean

[Quicksilver]

Had the same problem with my 87. It would buck hard enough to just about take your head off. Sometimes it would go away for a bit but it made me scared to stand on it. Everyone looked at it with no luck.

In a fit of frustration I decided to drop the engine and while disconnecting the plug for the injector harness I discovered that it was floppy loose. Turns out that normal vibration would bounce it up and down and when it started to buck it would bounce up and down. I strapped it down with a nylon tie strap. Has worked perfectly since then.

I would check that connector which is in the very front of the engine compartment, in front of the heater blower, right next to the driver's side shock. If there is any looseness to the connection it could be the problem.

If that isn't it I would check every electrical connection in the engine compartment starting with the ground on the intake manifold and the transmission grounding strap.

Good luck...

Wayne

[Joehand1]

OK Sean, I told you these guys are good.
Joehand1

[Imcoz]

I'm having a similar problem w/ my 88, it idles, runs, but under acceleation at 4100 rpms or there about in any gear, it just gets real rough and stops accelerating, it's not a bucking, but more like it's running out of gas or getting a poorly timed spark. I've checked the fuel pressure, replaced the main relay, plugs, wires, cap, rotor, fuel filter, the 02 sensor and checked grounds... no fix, not sure what to so next... Could you let me know what you find out via this post??? I will post an answer from Stephen Kaspar - PCA WebSite

[Imcoz]

Imcoz's answer from Stephen Kaspar - when I posted a similar problem on the PCA WebSite - I might suggest that you check the injectors. As these cars continue to get older I see more and more issues with the injectors and the harness. On several
occasions the tips have fallen off or the injector has simply failed. If either happens the pattern will cause fouling or inconsistent burn on that cylinder.
I have also seen the main injection harness with shorts which in turn will dump the injector and foul the plug. Have you taken a reading of the plugs? Is there a cylinder that is richer or leaner than the others? How about bank to bank?
If you have a cylinder than is fouling over the others check the injector. If you have a bank check the harness. There can be other things that will cause issues such as the HTS or even the
coil. You could also have a failing ref sensor. All of which have a procedure for checking. I would start by looking at the plugs. One thing that might help to be more accurate in the plug viewing is cut a plug at that RPM. In other words if you are at WOT and at 4k RPM shut the car off right in the middle of it and pull to the side. Pull the plugs and check them right there on the spot. The
reason you do this is as idle the car may be fine and give a false reading. At 4K where the issue occurs this is an accurate way of looking inside the motor so to speak.

Stephen Kaspar - PCA WebSite

[ssscarrera87]

Thanks or all the info,I wil look into these areas .
The one thing, this is happening at lower rpm just driving along light on the gas . it is not as violent as Wayne stated,and you can make it go away by raising the rpm, but I'm going to look into the injectors and harness
I will let you all know as soon as something happens
Sean

[SpeedracerIndy]

You might check for moisture inside the distributor cap. I have had that cause this same problem with a Carrera. It's a long shot, but worth a look.

[ssscarrera87]

I will have a look in the distributor
thanks for the idea

[john walker's workshop]

hang a new cylinder head temp sensor and DME relay on it before you spend a lot of time on it.

[evill]

Originally posted by john walker's workshop
hang a new cylinder head temp sensor and DME relay on it before you spend a lot of time on it.

COZ, John Walker was the guy I told you about yesterday. (John, I told him to post his question here and hope that you would answer )
I would try the Cylinder Head Temp sensor as he suggests. theres not much left that you didn't try.

Eddie

[ssscarrera87]

John
Thank you for the info, I will give that a try
and thanks to all the other suggestions.I will post the results soon
Thanks to all
Sean

[Quicksilver]

Originally posted by Joehand1
OK Sean, I told you these guys are good.
Joehand1

It is only good if it is the right answer.
A really violent bucking would normally be electrical because that is the only way you can turn it on and off that quickly, so general electrical is probably out.

Less violent; I would still look at engine grounding straps (it's free to look!) but it would make me think of sensors or fuel. The injectors seem to be an obvious place to look except I would expect an injector problem to get worse as the RPMs rise.

It probably isn't ignition. A spark gap has a lower resistance in a vacuum which invariably means that the higher the throttle position-> the higher the cylinder pressure-> the harder it is for the spark to jump.

In the sensor dept... Has your car ever had the cylinder temp sensor replaced? It is a notorious problem on the early Motronic cars as pointed out by John Walker suggestion to start there. BTW it is easy to remove the old sensor by clipping the wire off but it requires a special socket to put the new one in. To make your own just get a deep socket and grind a slot all the way up one side of it so the wire can hang out.

Wouldn’t of thought of the DME relay but you should always keep a spare in the car anyway and it is easy to get at, under the drivers seat.

I would also check the Oxy sensor connection. If it was intermittant it could vary the fuel quite a bit.

Wayne

[ssscarrera87]

Wayne
I will be sending some time in the engine compartment
thanks for all the ideas
Sean

[ChrisBennet]

If you fill out your profile so people know where you live someone might volunteer to come and help you.

I would check that the head temp connector is secure. Its not enough to push on one of the connectors, you need to squeeze both sides of the connector to make sure it is seated. You'll probably want to remove the plastic heater tube that's in the way to do this.

The head temp connector is on the left side of the engine compartment below and to the left of the heater blower. There are 3 connectors in the same bracket. The head temp connector is the white one. I believe the old sensors use the engine for ground and the new ones have their own ground wire.

The new sensors have a very white wire sheath and they take a 14mm socket. The old ones take a 13mm socket I believe.

-Chris

[ssscarrera87]

Chris
Thanks for idea.I'm about to get to work and see what I can come up with
Sean

[Jeroen]

It's most likely the air flow meter (or flapperbox, barndoor, what ever you wanna call it)

The switch on it is probably worn out
This is because the RPMs at which the problem occurs are the RPMs at which the car is (or was) most driven, so the switch/contact sees most wear at that position

Check the contacts of the switch and see if you can clean it up or replace the whole unit with a new, used or rebuild one

cheers,

Jeroen

[ssscarrera87]

Jeroen
I already tried a replacement airflow meter it was not the problem.
But I think I have found the cure,I went over the three sensors on the flywheel,cleaned or replaced the connectors ,and now it is as smoth as silk at 2800 rpm and as nice through the power band,I can't wait for the break in period to be over so i can go up to the red line
Thank you all for your input ,there were many good ideas
Sean