Texbuckeye

Hey guys,
My 86 has been misfiring when you get above 2200RPM but only when it is hot. We have checked the timing, replaced the spark plugs and coil and vacuum lines and made sure they are all connected. It runs great until you turn it off and then restart it. Then it starts misfiring and running rough.

What do we need to check?

dmcampbell

Stock coil and what plug brand? Plugs gapped to .028? DME relay ok? If so reduce plugs to .025 and see if it helps. Mike

Texbuckeye

yes stock coil and bosch plugs. plugs were gapped to the recommended gap. How do I determine if the DME relay is ok?

Tom M'Guinn

Can you give more detail about when it starts to misfire? Does it happen whenever the coolant temp is at operating temp, or is it more tied to how long after the last start its been running? Do you have any kind of afr monitor? Is it always right at 2200 that it starts to misfire and does it matter if you feathering or flooring the throttle?

DME relay is usually on on/off thing, so poor running is rarely tied to the relay. Your problem sounds more like weak ignition (how are the wires, cap and rotor) or afr going out of what due to a bad sensor, fuel delivery, vacuum leak, etc. Tell us more about what's going land we might be able to help narrow it down...

Texbuckeye

Thank you guys for helping with this. Yesterday morning I went and drove it for 30 minutes pretty hard on the back roads and then stopped for gas. It ran great. When I started it up again it was misfiring bad again above 2200 RPMS. So I thought maybe it is computer related so I stopped and turned it off for 30 seconds. It didn't do anything different so I turned it off for 2 minutes and that didn't make a difference. I checked that broken piece and it is solid. So I don't think it was the coil. I think whatever the problem is is related to the car getting hot and it may be related to starting hot. When I started it cold this morning it ran great.

Oddjob

How old are the plug wires? Check for arcing (open the hood in the dark).

Tom M'Guinn

Weak ignition can cause sputtering under heavier loads, and plug wires are an often overlooked cause of weak ignition. Based on the description'' though, I'd lean toward a sensor or electrical connection that's going bad. Can you post a video of when it misfires it might help. In the meantime, you might start by confirming the wires, cap and rotor, and the tps, temp sensor, an AFM signals when tr the car is acting up. While its usually the down on the list of things to check, it might be worth checking your dme and KLR boards for cracked solder joints (or swapping in spares), since the nature of your issue feels like that kind of issue...

Texbuckeye

Thank you again Tom! I went and drove it again this morning and about 30 minutes into the drive without stoping it did it again. The temp guage had moved up to about 1cm from the top line. So I don't think it is not related to starting hot. I think it does it when it gets hot. Also it is not at 2200 RPM that it does it but around that point and higher. So as long as I shift early and keep it under 2000-2100 it doesn't misfire. If I press the throttle down to raise the RPMs it does it again even when parked.

It seems to be directly related to the car heating up as each time that has been when it started.

Texbuckeye

Here is a pic of the instrument panel. I noticed the Amp guage is reading low. Is that the potential source of my problem or a symptom of not being able to really rev above 2200 rpm?

SeanR

Here's a vid showing what it's doing.



I changed out the coil and Brad changed the plugs a few days ago. The issues seem to only show up after the car gets really warm. I have not gone in and tested the AFM or changed the Temp sensor yet. Brad can explain it all better than me but he had it over here for me to take a look at today. Have not had time to do a full on diagnostic on it and if we can save some time doing it via Rennlist, bonus.

dmcampbell

Have you cleaned the grounds at the top of the bell housing?

SeanR

Not yet, on the list.

Took it for a nice slow hot drive and it ran great until the temps got a tick above the white line and was there for a while. Once it hit real hot the engine did the sputtering/misfiring. Takes a while for it to happen so we are either heat soaking some electrics like the AFM or the Temp sensor is kaput. O2 disconnect made no difference and the voltage from the alt stayed at a constant 12.55v through out cold and hot operation while everything was running.

Have to replace an intake hose this week so will do the Temp II sensor at the same time. Will update as we get further in to it and will take you alls advice at the same time. Thanks guys.

Adker

Fix your charging system. It should be at 14V when the engine is running.

slivel

Concur. And it's a Volt gauge not an AMP gauge.

Tom M'Guinn

Any idea why the voltage gauge was reading so low? Did you confirm 12.5 at the alternator when the gauge was reading under 12? Ideally you'd be charging well above 13. Most likely that's unrelated, but you never know...

What's up with the intake hose that needs to be replaced? Which hose and why? Do you have vacuum leaks? Might that be related?

Before taking it apart, I'd be inclined to try a few of the more likely/easy suspects to see if you get lucky...

--pull off the TPS connector and see if it will run smoothly at higher RPMs (power/boost may be down, and idle may bounce, but does it run smoothly when hot above 2000 without the TPS?). If the idle contact is shorted closed, the DME will cut fuel flow to the injectors above 1600. The idle contact switch would be my leading guess if the car simply refuses to rev higher than 1600-2000. If it will rev higher but just runs bad above 2k then it could still be a bad TPS pot...

If you have a multimeter, check the TPS and DME temp sensor and AFM per clarks-garage. (If you don't have a multimeter, might be time to get one....

--check for good vacuum at the fuel pressure regulator, and that the FPR holds vacuum, and that there is no gas smell in the vacuum hose to the FPR.

--Can't tell from the video, but if it is lumpy after 2000 -- like one cylinder isn't firing -- trying pulling the injectors and spark plug wires one at a time to see if you can tell which cylinder isn't firing (by seeing if the motor runs the same with or without the injector or plug on one cylinder).

--Give the fuel injector wires and speed/ref wires a good shake while it's sputtering to see if any of the connections might be obviously bad. (Can't confirm they're good that way, but you can sometimes find a bad one that way.)

--take a look at that cap and rotor, and try new wires -- or even try re-routing and/or wiggling the plug wires while it's acting up to see if you can affect the motor.