Renix ECUs, 90 & 89 swappable?

I'm trying to pin down a rough idle and low power problem in a 90 4.0 automatic.
I got an ECU from a 4.0 automatic at a pick-n-pull yard, 89 I think. Am I likely to do any damage by plugging it in? Can someone tell me if these computers are basically the same?

The 90 ECU has an orange and white sticker with these numbers:
S101155201A and 53007842

The 89 has a yellow and white sticker:
S101140203A and EF8953005428

Thanks

I would try it. Both are for the exact same application right? I would do it. I could be wrong though.

One should run the other well enough - there were some emission changes to the EGR system that may not be reflected in your '89's box and something to do with the cold program...

How are your injectors? (reason I ask is that the drivers in some older ECUs are not quite as tough as the end-of-production units) - that, and old bendix injectors tend to develop low delivery - something to do with pintle shaping/wear

I think the injectors are OK. I haven't checked their spray pattern or delivery rate, but the rig only has about 100K on it, and we ran some injector cleaner thru it shortly after the current difficulties started.
In short the symptoms are these:
Slow and very rough idle, sometimes stalls. Less power than normal on the highway, can't hold speed on long hills.
Very poor vacuum:
5 or 6 " at 500 rpm idle
10" at 1500 rpm
15" at 2500 rpm
I feel that I've checked every possible source of vacuum leaks. Can poor vacuum be caused by misfiring due to ignition problems, etc?

I'd have your Cat converter checked, it could be stopping up your tail pipe. My 88 XJ has 16-18 in Hg at 900 rpm idle that transmission in park.

You may want to take it to a dealer you trust and have them do a diagnostic on it. I recall my dealer mechanic told me if i had 12 in Hg then the top end & intake manifold needs cleaning.

Might be worth it for an hour of dealer labor to have them use those dealer grade cleaners and solve your vacuum problem.

Also on our XJs you have to change the CCV system every 50,000 miles per the owner's manual. The large tube and small tube in the valve cover are the components. Once I changed all of that I had a much better idle. If the connectors look oily then you need to replace them. Expect to spend the better part of a $100 bill for all 3 parts of the CCV system.

Also the vacuum harness for the front axle shift motor can be a source of a leak. The harness runs around $10 at the dealer, repalce it if you have the NP231 part time transfer case.

If it was me I'd change the vacuum lines I mentioned, then remeasure the Vacuum. There are tests to determine if a cat also good using a gauge, might want to find the instructions for that.

On the injectors if they are silver color and have a black part at the top where it ocnnects to the fuel rail then you have the OEM Bendix injectors. Smart money would have you change those out, the seam between the metal and plastic is know to leak and cause a fire.

Hope that helps.

Martin

Here are a few more details.
I replaced the muffler and looked thru the cat. The light thru the honeycomb looked normal. I have removed the manifolds and checked the gaskets, etc. and cleaned the throttle body. The idle control servo seems to operate under control of the ECU. The TPS voltages are to spec and they change smoothly. The EGR valve is not leaking and it operates as I would expect. MAP sensor operates but of course puts out a signal indicating little vacuum. I have checked and blocked off every source that could be letting in air, but still have lousey vacuum. The CPS signal is about .6V AC when cranking and the pulses look very reasonable on an oscilloscope with the engine running.
The strangest thing about the rough running is that it was intermittant at first. My daughter had it at college and it would be rough one day and OK the next. Now its rough all the time and I've been fooling with it on and off for a couple of months when I've had time. I'll try the other ECU probably tomorrow night and if it doesn't help, I'll do some more compression testing and start pulling the head...unless anybody has any more ideas.

Thanks.

You can check the cat by connecting a pressure gauge at the port on the EGR valve that feeds backpressure to a control valve that is in series with the vacuum line to the EGR valve. It's a small metal tube comming out of the base of the EGR valve. My '89 has this and your '90 probably does also.

"Once you've made your connection, start the engine and note the backpressure reading. Depending on the application, the amount of backpressure that's considered "normal" will vary. On some vehicles, backpressure should read near zero at idle, and should not exceed 1.25 psi at 2,500 rpm. Others can handle 0.5 to 1.25 psi at idle, but should have more than 4 psi during a snap acceleration test.

If you find a relatively high backpressure reading (say 8 to 10 or more psi), there's obviously an exhaust restriction that will require further diagnosis." This is from http://www.babcox.com/editorial/bf/bf19922.htm

My '87 has rather low vacuum, but not as low as yours. I did a leak down test and found that the #1 cylinder was loosing much more than the others. The compression check didn't show much difference from cylinder to cylinder as the leak down test did. The leak down test will also help you figure out where the cylinder is leaking. With the cylinder at TDC and pressurized, you can use a tube to listen in the intake, exhaust, and crankcase to diagnose the path of leakage.

Larry, when it's idleing, is the vacuum a steady low reading or does it pulse? If it pulses then look at a cylinder or valve being the culprit. If it's steady, then I would still say a vacuum leak somewhere..........????? Have you checked the hose routing to the brake booster?

Martin mentioned a possible shift motor leak......

Severely retarded timing can also show low vacuum. Maybe the distributor gear slipping on its shaft???

I have not noticed any pulsing, but that may be because of the long hose connection to my gauge. I'm pushing a 3 foot piece of hose on the port for the brake booster with an old valve stem in the other end to connect my AC gauge set. I'll try a short hose with a different gauge this weekend.
If the ignition timing is off, can I check it with a conventional strobe timing light? Are there timing marks on the harmonic balancer?

Larry, there is a notch on the balancer, that can be aligned with markers that are on the cover. The timing is not adjustable because of stops built into the distributor base, preventing it from being turned. However, you should be able to put a chalk mark on the balancer notch, and check to see if the timing is where it should be and if it's advancing properly.

TDC is the second mark CCW from the right (looking downward). If you look closely, it'll have a 0 at it, but it's hard to see......

Well, I think I'm making progress. The trouble is apparently in the EGR system, tho I swear that I checked it carefully a month ago.
I tried checking the exhaust backpressure as suggested by PaulJ. I hooked my old R-12 AC gauge to the small metal tube that comes off the side of the EGR valve. When I fired it up I was rather surprised to see the gauge read vacuum instead of pressure. I decided that this meant that the EGR valve was seriously messed up and started removing the air box etc. so that I could get to the valve and take it off. After I took off the top of the air cleaner and disconnected the large vacuum connector, I decided to start it back up to check exactly how much vacuum there was on that pipe. I plugged a couple of small openings at the connector and started it. I nearly fell over when it ran smoothly for the first time in months. It "idled" at about 1300 rpm with no sputtering and the vacuum on the manifold was about 18 or 19".
After reconnecting the big vacuum junction it went back to idling roughly at 500 rpm, so I fooled around until I found that disconnecting the vacuum line to the EGR solenoid and vacuum servo made the engine run well. The EGR valve was being held wide open all the time.
This morning I proved to myself that the solenoid is bad. It makes a slight noise when I apply 12V, but it lets vacuum go thru all the time. I believe there is one at my favorite pick-n-pull recyler, but they are closed today. So next weekend I'll get back at it; the basic idle speed seems way too high now, after all the messing around I did earlier with the throttle body. We'll see.
Thanks for all the suggestions.