83 245t int. low power

Problem: when driving the car, I can intermittently feel the engine breaking up under any amount of throttle, with no particular pattern other than it happens most frequently in boost or more than 50% throttle. Usually, it has the most power and behaves for the first 5-10 minutes of driving. I can be cruising along at consistent throttle at 65 and feel the engine lose power, then pick back up as normal. It feels like a quick, light misfire that's consistent across all the cylinders. It sounds almost like compression with no ignition, but thats really just a guess. If I increase throttle, it will stay flat until without warning it picks back up.

History: The car has a new WUR, JY fuel distributor, confirmed working frequency valve and pressure switch (blue guy on firewall) and new fuel filter, cap and rotor and plugs. All sensors in the head replaced with new when I was reviving her from sitting. She just passed smog well under all limits. Confirmed pressure switch increases dwell when it sees pressure. I cleaned all the injectors in a bucket of gasoline (which become substantially dirtier after they sat submerged overnight) and she idled better with them installed. I had to run a new ground from pin 16 at the ecm to wake up the FV initially when I was getting the car running, but that wire is still in good condition and terminates on the firewall. Will try moving this, maybe right to the battery.

My suspicions: The FD shaft is binding or otherwise misbehaving internally, this was an issue I had with the original FD that came with the car. I have a k-jet fuel pressure testing rig but I'm not sure what numbers to look for, or if there is a problem with the fuel distributor, what I can do about that besides have it rebuilt or look for another known good. Or, the FV is intermittently losing signal and cutting fuel in boost, and my mixture is going too lean, but then why is it also breaking up at light cruising throttle?. May try to rig up a dwell meter that I can monitor in the cabin. Any other obvious tests I can look into? I've only really gotten into pre-obd2 stuff in the past year and a half, and I'm sure there are some obvious shortcuts I am overlooking. The way the power goes away and comes back also feels like it could be a spark timing issue.

That also could be the fuel pump relay having issues. I'd take a good look at the relay and make sure it isn't getting too hot and melting stuff inside. Sometimes you can reflow the solder connections inside the relay. An intermittent main fuel pump can also cause this symptom. I'd put the kjet fuel testing gauge on there and see how stable the system is.

I replaced the original dead relay with new when I got the car running, but I need to check fuel pressures. I made a tool to test, but I don't have service info on what pressures I should see.

Diving today, it breaks up through the revs on hard accel, or pretty much at any throttle over 4k or so.

breaking up under boost can be spark blowout. That can happen from plugs that are worn out or have too large a gap.

There are three pressure measurements. The system pressure should be around 65-70psi. The control pressure with a cold engine should be around 29psi and rise as the engine warms up to about 54psi. The rest pressure is the decay of pressure with a hot engine that is shut off and it should have about 45psi after sitting about 20 minutes. These are approximate figures I am quoting but should get you in the ball park.

Checked plugs and they?re good ngk bp7es with normal looking color. I was expecting to see one or two cylinder lean or rich given how much it pops and breaks up. Hmm.

Replaced injectors just cause I found a good deal on some new ones, and didn?t know the history on mine. No change, that?s not too surprising as the issue is worse when the cars hot.

Checked all engine timing and found all marks line up, checked ignition timing and it was dancing around *25 at idle (??). I swore set this a few months ago when I was trying to get the car past smog. Set back to 12, the car has no power, and still pops and has no power. Really, really, really losing my patience with this car. Going to take a few weeks off and just find the f*cking service information so I?m not going in winging it. Since everything is new besides the fuel distributor, that?s what my moneys on, but then why do all plugs show good burn? Time for a break.

I did check compression and get 150 across all cylinders. Valve adjustment was done when I got the car. Fuel pump does buzz a bit but getting 82psi or so after the filter.

adamdrives said:

Checked plugs and they?re good ngk bp7es with normal looking color. I was expecting to see one or two cylinder lean or rich given how much it pops and breaks up. Hmm.

Replaced injectors just cause I found a good deal on some new ones, and didn?t know the history on mine. No change, that?s not too surprising as the issue is worse when the cars hot.

Checked all engine timing and found all marks line up, checked ignition timing and it was dancing around *25 at idle (??). I swore set this a few months ago when I was trying to get the car past smog. Set back to 12, the car has no power, and still pops and has no power. Really, really, really losing my patience with this car. Going to take a few weeks off and just find the f*cking service information so I?m not going in winging it. Since everything is new besides the fuel distributor, that?s what my moneys on, but then why do all plugs show good burn? Time for a break.

I did check compression and get 150 across all cylinders. Valve adjustment was done when I got the car. Fuel pump does buzz a bit but getting 82psi or so after the filter.

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Sounds like your timing belt is off a tooth at the crank.

That's what I was hoping, but all the marks lined up. I'll have to check again tomorrow but this didn't look like the later style crank pulley with rubber that can slip.

Lining up the crank mark is not intuitive since the mark is off to the left from vertical a little when the rotor is pointing at #1, and the belt has to be wrapped around the crank gear a bit to line up the marks, then it pulls away as you wrap the other pulleys and tensioner.

Also it's easy to miss the intermediate shaft alignment due to the tilt of the engine so pay close attention to the groove / mark in the back metal surround by placing an awl / nail / wire in the groove to bring the mark straight out to the dot in the edge of the intermediate shaft's pulley wheel.

Typically when the crank gear is off turning the distributor won't quite get it back to spec, so I suspect the intermediate shaft alignment / timing.

Good luck and don't forget to re-tension the 17mm tensioner bolt with a deep socket after a couple hundred miles.

adamdrives said:

That's what I was hoping, but all the marks lined up. I'll have to check again tomorrow but this didn't look like the later style with rubber that can slip.

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"with rubber that can slip"... you're thinking of the harmonic balancer, yes? Good thought. Even if you don't expect it to be off, it certainly can't hurt to verify that all the marks are pointing to where you think they're pointing.

The notch in the washers around the crank gear lined up with the marking on the block. Using a mirror I verified the aux/dis gear was in time as well. As far as markings show, everything is in time. I'm going to see when the piston hits tdc with a boroscope today for a sanity check.

I remembered last night the first time I noticed popping was after putting a K&N air filter in. Going to double check everything on the airbox/FD/meter plate and maybe put the original paper filter back in.

Nothing out of sorts on the airbox.

Hooked the car up to our old Sun analyzer. One cylinder had an irregular waveform, but I couldn't figure out how to select cylinders, and then lost the waveform, and my patience. So, seems like there is an ignition issue, or the machine is old and not working properly. The car has a new Bosch coil and cap and rotor. Maybe it's the ignition module or distributor? How abruptly the car comes in and out of power does feel like spark or timing.

Spent some more time on the analyzer today. Cyl #2 had an irregular waveform, shooting way too high on the firing line. Put a tester cap and wires on it and it runs smooth with uniform waveforms. Took it around the block, smooth power with no breaking up. I replaced the cap and rotor and put new wires on the car when I got it, but obviously the cap or wires were bad. The wires didn't have any breaks or rub marks in them, and the cap didn't have any scorch marks and wasn't cracked. Happy it’s running smooth now, but still feels like its lower on power than it was before the intercooler install. Did a quick launch and when I came back I was surprised to see two rubber stripes on the road. I thought only 740s had a limited slip from the factory.