[Richard55]

I went to take the 93 SWB out for a drive today and a weird problem cropped up. It will start fine and after idling well for a couple of minutes it will start this oscillation where it seems like it is going to stall and drops to about 500 rpm, then recovers and comes back to normal for a few seconds, then repeats. If I shut the motor off and restart it the same pattern will happen. It starts great then gets into the oscillation. Adding some throttle does not seem to make a difference in the oscillation except making it recover to a higher rpm when it jumps back up.

After messing with it for several minutes like this the check engine light came on with a code 17. Would the TPS really make it oscillate like that or could it be something else? I was recently facing some issues with the advance on the timing at higher rpms. I replaced the distributor and afterward it was running better than it ever has across a variety of driving conditions for about a week until this happened. Unfortunately I don't think the TPS is interchangeable with my 95 to make for a quick and easy test. If I want to test it with multimeter, what is the test procedure? Since I recently messed with the distributor, is it more likely that I messed something up with that and maybe the TPS is ok?

 

[KK88RRC]

http://www.v8engines.com/carbs-2.htm about 2/3 down the page you'll find testing info. TPS in the 93 is not adjustable unless you elongate the screw holes.
I would give the stepper motor a good cleaning too

 

[Richard55]

Good idea on the stepper motor (IAC), it would definitely lead to a bad idle. My cheap multimeter wasn't working so I decided to try everything else I could think of until I can borrow one from a friend to check the TPS voltage. Since I had another truck sitting next to it in the driveway, I decided to swap over parts one at a time that were the same.

1. I took the IAC out of both trucks and put the one from the good 95 in the problem 93. It made no change to the behavior.
2. Thinking along the lines of everything that would affect how it runs I then took the MAF from the good 95 in the problem 93. It also made no change to the behavior.

I then put them all back the way they originally were.

3. I then pulled the distributor cap to make sure nothing looked bad on the inside or with the rotor. Everything looked perfect inside so I put it back. After this it seems to idle ok now.

It still has the TPS code 17 so I'll check the voltage on that next and then decide it needs to be replaced also. I suspect it may be ok though since it is idling well again and may be ok if I just clear the code.

 

[RRLondon]

It looks that the IACV circuit is working correctly to me. But it seems that it's trying it's best to control something else that's giving you variability, and just about containing the situation to keep it running.
The TPS as I understand it has no adjustment because on this type the ecu takes a reference reading on startup of TPS output voltage to act as baseline. Effectively it "calibrates" at every start and then works it's operating range from that set point. That should always allow it to run at optimum without manual adjustment.

The base idle setting should prevent the rpm dropping below the mechanical point at which it has been and give a lower level "catch net" that effectively overrides the ecu and IACV. This should not be reset while a variable running condition is present as it will incorporate that error into setup and make you chase your tail.

It looks more related to a cylinder or more dropping out sometimes at idle (the base idle can't cope with this) and moving the HT leads etc could have given you a clue. Try when cold starting it and shutting off after 10secs, then feeling each exhaust manifold outlet to see if you've got any cold cylinders initially. This may isolate you a cylinder to give you a suspect.
Also try moving each HT lead while running to see if any are arcing to give you some intermittency, in effect switching one cylinder on and off.
I say this as when you've disturbed the distributor cap it seems to have changed something.

When I put the cap on I set it down into place and wriggle it by just as much as it'll move to get it sitting down competently, which you may have done at last intervention.

Check carefully the distributor vacuum tube for leaks too.

In summary it looks like you've a cylinder just fluctuating in and out of operation and the control trying to mediate, with just about, success.

 

[Richard55]

Unfortunately the issue came back so messing with the cap did help, but didn't permanently fix anything.

Thanks for the ideas. Since it runs great for about the first minute it could definitely be something failing once it starts to heat up.

I think I have some spare, new vacuum line that may be small enough diameter. If so I'll just replace the section between the distributor and the intake plenum to eliminate that possibility since it will be quick. I have an inline spark tester that I could use one plug at a time, but if I'm pulling one end of each wire and putting it back it may be a good idea to just proactively replace the wires. It was running really badly with the bad distributor so maybe it damaged one or more plugs.

 

[RRLondon]

If you doubt the plugs then that would give a good start point to go with, although they generally work or don't so normal starting would suggest they are not the problem. If you replace, don't open the gaps on them, run them tight on correct minimum spec.

I'm always reluctant to throw HT leads at them as diagnosis except for the main/king lead. This will obviously affects all system thereon downwards and partial fail here is worth eliminating first line to prevent you chasing all the others down.

The change as it warms, I'd split into two parts as hardware (leave aside initially) and conditions. Usually with cold start it's fairly easy to get it going with ecu giving richer conditions to light it up. That quickly changes as temp sender reports and mixture is backed off.
As it gets leaner then spark gap resistance goes up at the plugs, that means performance of HT leads comes under more demand and will expose any weakness as temp rises.

As a guess, then king lead first to assess and work from there.

 

[rangerovering]

Set your timing correctly at 6 degrees and clamp the hose for the idle control valve, the engine should idle at 525rpm +/- 25rpm. Check this first because it will save you a lot of time messing about


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[Richard55]

I finally got a chance to mess with it for a couple of hours today. I pulled the plugs and they looked pretty bad. I have no idea when they were last changed so I went ahead and changed them along with the wires, cap, and rotor since they are all maintenance items I would do eventually anyway. No change to the behavior with those although it does idle better and the rpms don't drop as low as before, but otherwise the behavior is the same.

I should have a couple more hours tomorrow so I'll swap in the coil and ignition amplifier module from one of my working trucks to eliminate those possibilities.

 

[Richard55]

This morning I tried checking the temperature of the headers near each exhaust port after running for ~20 seconds as RRLondon suggested and all get hot quickly so I believe all cylinders are firing after starting. I also used the coil from one of the trucks that is running well and there was no improvement. I was hoping to try the ignition amplifier from one of the 95's also since it is easy to get to once I had the coil out, but on the hard dash it is on the distributor and I couldn't find a way to remove it without pulling the distributor.

I think I'm just going to bite the bullet and take it to my mechanic.

 

[Richard55]

The diagnosis was the ignition amplifier and pick up coil. Murphy's law that it was two parts that I could not easily swap over from the 95. I almost did the ignition amplifier when testing, but stopped because the 93 had it on the distributor and I couldn't simply plug in the second connector without pulling it from the distributor.

 

[Range123]

Well worth considering relocating the ignition amplifier away from the dizzy itself. I had many issues along the same lines and found the amplifier was over heating. relocated on the opposite inner wing. happy days.

https://simonbbc.com/rover-v8/ignition-relocation/

I also had similar symptoms due too a leaking vacuum hose from the brake servo - last place I would have looked!!