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Intermittent skip?

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Old 04-14-2018, 10:46 AM   #1  
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Intermittent skip?

OK, I'm losing it. 92 fb 305 TBI has had a phantom CEL for two years, never stays on more than a minute or two, usually on the highway. Recently, it developed a noticeable skip. It bucks a little under light load at 30 mph (3rd gear converter lockup), and you can feel it stumbling on the highway a bit. Recently replaced injectors, rotor, cap, coil, and just this morning the ignition module. Put a timing light on any plug wire and I can see the occasional miss. Plugs aren't very old, wires are 10 years old but nothing obvious wrong. What else could it be?
It seems to get worse when it's warm, but that might just be the high idle hiding some of the problem. The car still only has 82,000 miles. Most of the vacuum lines are original, so something could be leaking a bit, but that doesn't seem to match with the timing light skip.

Any ideas? I've got a few other mechanicals to fix (door hinge bushings, ball joints, and a window motor), and I'm getting a price on some floor, wheel well, and body repairs. This has been my daily driver for 10 years after buying it from the original owner, but now I'm also looking for another rust free low mile one to drive if this one is going to cost a lot of time or money to stay reliable (or keep aggravating me with what seems like it should be a simple repair). I can't take the time to tear it down myself--too many other projects.

Thanks!

Last edited by Matt T; 04-14-2018 at 10:47 AM. Reason: typo and add thanks
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Old 04-14-2018, 11:04 AM   #2  
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Re: Intermittent skip?

What code is being stored by the ECM, if any? The CEL should have generated something, unless it was flashing instead of remaining on.
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Old 04-14-2018, 03:54 PM   #3  
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Re: Intermittent skip?

The first thing I would do is unplug the ECM and then plug it back in again after inspecting all the contacts.

If you have a crusty connection, its gonna do the bucking-bronco thing.

I replaced my ECM (after parts-cannoning damn near everything else to no successful effect). After replacing the ECM, the stuttering and bucking bronco thing was permanently gone. In retrospect, I doubt the ECM itself was the problem. I now suspect it was just the nearly 30-years crusty connection. The action of unplugging it, and re-plugging it to a new ECM probably knocked-off enough crud to restore a reliable connection.

Consider giving the harness-plug and ECM-pinout-jack a good cleaning while you have it unplugged.

It won't hurt to unplug, and re-plug the PROM chip, and clean those connections too while you're at it.
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Old 04-14-2018, 06:26 PM   #4  
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Re: Intermittent skip?

Working tech support over the phone at Gateway for four years, you wouldn't believe the hardware issues that were cured by reseating cards and cables, and this was on new PCs less than a year old.

But yeah, pull the codes. A light should store something. an intermittent problem often gets worse until it finally sets a code. BTDT with the knock sensor, fuel pump relay, and VSS failures. None of the above set a code with the earliest symptoms of trouble, but set a code awhile later.
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Old 04-14-2018, 06:41 PM   #5  
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Re: Intermittent skip?

Highway CEL on a TBI 3rd gen is usually an EGR issue. Now, this is an educated guess, so do pull the code(s) first.

Second guess would be a silicone poisoned O2 sensor. Again, a code will tell the tale. Or at least point you in the right direction. Can use a scan tool to check the BLM (long term fuel trim), if low it points to the O2 sensor.

If the BLM is high points to an EGR issue or a failing fuel pump. Note that there isn't a failing fuel pump code, so need to take everything into consideration.

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Old 04-14-2018, 08:17 PM   #6  
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Re: Intermittent skip?

Thanks for the replies. The OBD1 only says 12--which means the test light is working and nothing more. I'll try unplugging the computer and looking for bad connections to clean up. It's never been moved or unplugged so hopefully that's it. It has been damp over there, first for a heater core years ago and more recently a firewall leak below the blower motor. Maybe some pins are corroded.
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Old 04-14-2018, 08:23 PM   #7  
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Re: Intermittent skip?

You said 92 305 TBI. I am going to assume yours has the same ignition as my 95 305 TBI truck. Two easy things to check: 1) cheap rotors can arc out to the center mounting post in the distributor. You already replaced yours, so that's probably not it. If it were an issue, you have to look for a hair-fine arc line with an eye loupe. The spark goes from the coil to the coil connection on the distributor cap, but nothing comes out to the plugs when it completely fails. It is intermittent before that.
2) The pink and white wire coil jumper harness is a known failure point. Remove it and check for corrosion and test it with an ohmmeter.
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Old 04-15-2018, 05:40 AM   #8  
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Re: Intermittent skip?

I know you said you replaced the coil but I would do a quick primary and secondary resistance check, just because it's new doesn't mean it's good. Coils tend to crap out when they get warm. Also check any ground wires related to the ignition. I like using this stuff on all the wire connectors.http://www.acehardware.com/product/i...nt=Catch%20All
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Old 04-15-2018, 08:06 AM   #9  
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Re: Intermittent skip?

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Originally Posted by Matt T View Post
Thanks for the replies. The OBD1 only says 12--which means the test light is working and nothing more...
Technically, that is not completely correct. 12 means missing distributor reference pulse. That always occurs when the engine is not running, so some people erroneously interpret that as "normal" when it can actually be a DTC set while running.

Poor performance of the pickup coil, corrosion on the reluctor poles, poor connections of the pickup coil or HEI module, or anywhere in the primary ignition system can cause that.
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Old 04-15-2018, 12:57 PM   #10  
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Re: Intermittent skip?



The two “plugs” that attach the wiring harness to the ECM are each comprised of the black plug that contains all the wires and a blue clip that helps to secure the male and female connectors (insert “barbed *****” joke here).

Be prepared for the blue “barb/hook” clip to crumble when you flex it. The black plug that contains all the harness wires seems more durable.

When the blue plastic destructs on contact, take heart that you are not the first to suffer this inconvenience. Hawks will sell you a set new blue bits and black plugs.


http://www.hawksmotorsports.com/ecm-connectors-85-89-tpi-88-92-tbi/


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Old 04-16-2018, 10:38 AM   #11  
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Re: Intermittent skip?

...So I may completely disable the car when I disconnect the ECM to check the terminals?! How insecure are the plugs without the clips? (If they self-destruct is tape a temporary option?)

Now I'm glad it turned cold & rainy yesterday so I didn't already try this.
I don't have a spare car to take to work, so waiting for new clips from Hawks if they break really isn't a good option.

Thanks
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Old 04-16-2018, 12:25 PM   #12  
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Re: Intermittent skip?

I'm pretty sure you can just throw away the gray and blue clips, and still get connection.

The black plugs fit pretty tight. Problem is, they engage the ECU from the bottom. Hot/Cold cycles plus vibration, I'm sure the black plugs will work their way loose without the gray and blue clips.

There are ways to improvise with zip-ties or some such if you wer really in a pinch with loose ECU/harness connectors. But, in the grand scheme of things, I think we should be grateful that Hawks has the unique part we need, even if Hawks is demanding to make a profit on the sale. I'm OK with that.

I'm sort of a zealot at this point. I have (at least one) spare ECU, and I'm working on getting up to speed on copying the EPROM chips, and re-wiring the circuit board in the ECU. The tools and parts aren't free. But they really aren't that expensive by comparison. What would you rather have? A whole new set of soldering tools, or one night at the Sheraton by the ocean somewhere? Its a no-brainer for me.

The only thing I find expensive about playing with these cars is my TIME. Like it or not, time is absolutely irreplaceable. So long as I'm having fun, the time goes by without pain.
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Old 04-16-2018, 01:23 PM   #13  
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Re: Intermittent skip?

Glad there's a way to rig it, if needed.
Time and money are always the killers. I can't work full time, maintain/update our house, manage everything else we do, and spend too much time fixing my daily driver. I've also had plans for 2 years to switch it to 5-speed, but I'm not wasting the time until I'm sure everything else is solid and working properly.

The car project that is expected to take my time is building my 71 Camaro for our son to drive.
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Old 04-16-2018, 02:02 PM   #14  
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Re: Intermittent skip?

Quote:
Originally Posted by W.E.G. View Post
I'm pretty sure you can just throw away the gray and blue clips, and still get connection.

The black plugs fit pretty tight. Problem is, they engage the ECU from the bottom. Hot/Cold cycles plus vibration, I'm sure the black plugs will work their way loose without the gray and blue clips.

There are ways to improvise with zip-ties or some such if you wer really in a pinch with loose ECU/harness connectors. But, in the grand scheme of things, I think we should be grateful that Hawks has the unique part we need, even if Hawks is demanding to make a profit on the sale. I'm OK with that.

I'm sort of a zealot at this point. I have (at least one) spare ECU, and I'm working on getting up to speed on copying the EPROM chips, and re-wiring the circuit board in the ECU. The tools and parts aren't free. But they really aren't that expensive by comparison. What would you rather have? A whole new set of soldering tools, or one night at the Sheraton by the ocean somewhere? Its a no-brainer for me.

The only thing I find expensive about playing with these cars is my TIME. Like it or not, time is absolutely irreplaceable. So long as I'm having fun, the time goes by without pain.
The clips have prongs that hold the pins fwd. the pins have locking tabs so they have to be pushed down to release them from the connector.
If you need a ecm you can look on eBay or check the classifieds here and swap over your prom. The ecm is a service number 1228746 and was used in 89/92 tbi .
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Old 04-18-2018, 05:10 PM   #15  
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Re: Intermittent skip?

I wrote a (somewhat long) piece about the "clips" that hold the wiring harnes in place with the ECU.

https://www.thirdgen.org/forums/tech...-computer.html
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