Renix issue?

How ominous to put that in the title, I'm sure the renix gurus are saying ah geez here we go again. I've searched and read the renix files from 1st to last, but nothing seems to fit. I've got a 90 laredo(aw4, np231, and of course the 4.0 renix) My problem begins usually at highway speed, she will run all day long just as long as you don't have to slow down or stop. Once you do, she coughs, backfires and has a general loss of power. If it drives fine and you turn it off it takes an abnormally long time to start. My initial impression is heatsoak, however it had suffered from high idle particularly in the winter months. A 2ga ground from bat. to block seemed to clear it up and cleaning the remaining grounds didn't hurt either ( told you I read them). Just wondering if any1 may have some insight into this. Also can those damn plastic vacuum lines be replaced with regular vac hoses? Mine are snapping and popping all over.

Fuel pressure regulator sticking???? You could test that with a gage on the fuel rail.

I will try it, ive checked the pressure and it was within specs, the problem is intermittent it doesn't always do it. The trick will be catching it as it is doing it and throwing a gauge on it.

The Renix has to "spin up" to 300+ rpm before the CKP will send a signal to the ECU so the engine can start. Weak battery/starter and soleniod or bad cables and grounds will agravate the issue.

You can try "priming" the fuel system by turning the key to the ON position where the fuel pump will run for 2 seconds, wait 3-4 seconds then turn the key OFF, then repeat one or two times.

You can add a check valve to the fuel system after the fuel filter. I have installed a 3/8" valves purchased from http://myworld.ebay.com/ebaymotors/sam_the_diesel_man/ on about 6 different FI vehicles having long-crank start issues.

I appreciate the help, the thing is only difficult to start after it has come to temperature and then turned off and left to sit for a short time say at a gas station. fill up, pay, and try to start. That is why I thought maybe heatsoak. I'm in the process of upgrading all cables to 2ga as I know that when wires heat up they become resistive. I felt that the two problems were connected. When I run down the road till it comes to temp then catch a stoplight or something that is when it falls on its face. coughs, bucks, backfires, almost sounds like it does so through the intake. Ive checked the dreaded tps and voltages seem spot on, tho now that I think of it I haven't checked it at temp. been this way for a year. It may go a week without a hiccup then a 40 mph rodeo ensues and then be fine for another week.

Renix are not readily susceptible to the heat soak issue.

You should replace all the old vacuum lines, you can use rubber or buy the sets from Rock Auto: DORMAN Part # 46004 {#53004868}
Vacuum Harness; Rear
$17.06$0.00$17.06
DORMAN Part # 46003 {#53004838}
Vacuum Harness; Front

I doubt its heatsoak, but I have seen heatsoak issues fixed by wrapping the #3 and #4 injectors with spark plug insulating material.

Part of you issue is most likely vacuum related if you have cracked and leaky lines. I re-plumbed my 90 and got rid of all of the plastic that I could. the line from the MAP to the TB is the only one left and its new. I'm running a aftermarket valve cover that is for a 1996 XJ and it works fine.

I will try to get the vacuum issues resolved. The plastic lines really are in horrible shape. If I do go the rubber route, does inside diameter matter? I mean other than fitting them to their location. The plastic lines are really small id, would a larger id rubber line hurt performance. I would love to replace with oem plastic but they are very fragile and Im afraid I'll have to replace them every 20k miles. Thanks for all the help and info.

That back CCV hose has a metered oriface, so its supposed to move a certain amount of air. It's possible it's meant to work with a certain size hose. However, my oriface is drilled out and the hose is a bigger fuel line hose, and I was able to get it to idle at the right speed. MAP hose size probably matters, I think that's the only original hose on mine. I spray TB cleaner through it to clean it. You might want to keep the original plastic on fuel pressure reg as well, I'm still using plastic there too, tho it's not the original one. The plastic is good because it does not collapse, tho fuel line works well for the size I used on CCV. You just want to avoid that really soft rubber stuff.

You don't see much of it in my pic but some of my vacuum hose is actually "2-cycle weed whacker fuel line", you can pick it up at any lawn mower repair shop. Don't buy that cheap stuff at Advance Auto or AZ. The 2-cycle fuel line is made of silicon and will hold up much longer than the plastic or the rubber hoses.

Vacuum line simply needs to be vacuum line - it's moulded so it won't collapse when a vacuum is pulled on it (and the construction is therefore different from the typical pressure line - the walls are usually stiffer.) ID isn't critical, as long as you can get an effective seal at each end.

You can also pick up the entire vacuum harness from a NAPA - it's sold under their Echlin brand. I don't recall the part numbers offhand (there are three,) but I've posted them here before. Expect to pay about fifty bucks for the whole setup.

joe_peters is essentially correct, save one small detail. The CKP is used to read crankshaft speed, and the ECU won't fire the ignition or FI pulses until it reads 300rpm at the CKP. I haven't found out why as of yet, but I'm inclined to think it's to get oil circulating (the basic specifications for the RENIX control system were laid out by AMC and Renault, but the system itself was built by Bendix/King - and Bendix/King also does avionics. Such reasoning would be in line with avionics design mentality.)

I'd advise doing a "function check" of the TPS using an analogue ohmmeter - I've detailed how that should be done here several times before. When you use a DMM/voltmeter, you're checking adjustment - but that won't turn up a "flat spot" in the TPS response. I've run into that a couple of times, and the only way to find it is by using an analogue ohmmeter and "sweeping" the sensor while watching the meter's response.

Since the Renix ECU doesn't store any data, it has no idea what the crankshaft position is to start the ignition and fuel injection sequence. So it needs a few revs to get the signal from the Engine Speed Sensor (or CPS/CKP, yes, it has a lot of names) to determine when each pair of cyls are at TDC (1&6, 2&5, 3&4). Then it uses the Sync Signal Generator (or Camshaft position sensor, yes, it has a few names too), to determine when #1 or #6 is on the exh stroke. Then it also knows which injector to fire.

Saudade said:

Since the Renix ECU doesn't store any data, it has no idea what the crankshaft position is to start the ignition and fuel injection sequence. So it needs a few revs to get the signal from the Engine Speed Sensor (or CPS/CKP, yes, it has a lot of names) to determine when each pair of cyls are at TDC (1&6, 2&5, 3&4). Then it uses the Sync Signal Generator (or Camshaft position sensor, yes, it has a few names too), to determine when #1 or #6 is on the exh stroke. Then it also knows which injector to fire.

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True - but it could get that information long before the crankshaft hits 300rpm - yet the 300rpm trigger is something I've seen specifically mentioned in a few different sources.

If it wasn't an issue of crankshaft speed, it would say "valid CKP and SYNC signal" instead, I'd think - so it's probably trying to get the engine assembly turning, which means the oil pump is turning, and everything's getting moving before it fires.

I understand what you're trying to say - and that's why I'm still trying to track down the last patent holder for RENIX (got it tracked to ACTIA Corp.) and see if I can get any information out of them. This system hasn't been installed in anything in nearly twenty years now, so I'm hoping I can get through to them and get something back - finally.

The Renix manual does say that the ECU determines the correct cranking speed (approx 300 rpm) yada yada, but doesn't offer any reason for this particular speed. Could be just that it's a lot of stuff to get moving, overcoming inertia, friction and such. Maybe they threw a dart at a board and it hit 300......

Thanks again for all the help. I'll have to pick up a cheapo analog MM. It could very well be a flat spotted tps. I drove it tonite, and when it started falling on its face, instead of more gas I let up on the pedal and to my suprise the rpms came up. It still backfired a little and bucked some but not nearly as bad. AZ has a silicone vacuum line kit. How long does oem plastic generally last? I know that is a fairly subjective question, but 50 bones is 50 bones to a laid off guy.

xjtrailrider said:

I doubt its heatsoak, but I have seen heatsoak issues fixed by wrapping the #3 and #4 injectors with spark plug insulating material.

Part of you issue is most likely vacuum related if you have cracked and leaky lines. I re-plumbed my 90 and got rid of all of the plastic that I could. the line from the MAP to the TB is the only one left and its new. I'm running a aftermarket valve cover that is for a 1996 XJ and it works fine.

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how did you do away with that Renix vacuum line mess? im having a similar issue as the OP.

i ordered the rock auto vacuum line setup today. im wondering how to clean up and get rid of some vacuum lines myself....

Ok renix gurus, checking for a flat spot in the TPS I decided to probe voltages. Connector disconnected A-B:4.63v and B-D:.83. With connector connected and checking on the sensor side of the connector. With neg lead on battery neg I got pin A:5.55v B:4.68 and on D:.81v. .81 seemed high so I ran a ground to the battery from pin d on the sensor side of connector the voltages went Pin A:4.75v Pin B:4.01 and pin D was on the mv side of the scale. Prior to all of this with the connecter connected and probing A-D:4.71v B-D:3.98. Can someone give me a heads up on how these voltages look and whats up with the fluctuations By my calculations that came out to about 84% so I know it needs adjusting.

jujuscisyn said:

Ok renix gurus, checking for a flat spot in the TPS I decided to probe voltages. Connector disconnected A-B:4.63v and B-D:.83. With connector connected and checking on the sensor side of the connector. With neg lead on battery neg I got pin A:5.55v B:4.68 and on D:.81v. .81 seemed high so I ran a ground to the battery from pin d on the sensor side of connector the voltages went Pin A:4.75v Pin B:4.01 and pin D was on the mv side of the scale. Prior to all of this with the connecter connected and probing A-D:4.71v B-D:3.98. Can someone give me a heads up on how these voltages look and whats up with the fluctuations By my calculations that came out to about 84% so I know it needs adjusting.

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Sounds like the ground was bad on the TCU side of the TPS if I understand you correctly. You just need to recalibrate it on the TCU side of the TPS per the 89 FSM (See the RenX Files thread for links to the procedure) now and then pull the battery cable for 10 minutes to clear the Renix computer memory (ECU and TCU both!) of the old TPS idle values.

Thanks Ecomike, I was hoping you'd stumble across that post. It was weird, when I saw that voltage on D I ohmed out the ground and it came back 0 but ran the wire anyway and the voltages dropped on all pins. Too bad to cuz I was diggin that 5.55v...knew it was to good to be true.