Ignition problem with 3100 to 3500 swap w/ no code

[kerno]

I am nearly done with this swap and have documented the things I've done to make the swap work. But I really wanted to get it running before I post the step by step on the swap. Here's the current problem: The car really tries to start but it acts like I have the plug wires scrambled. It seems like it will catch and run weakly until a plug fires that is too far advanced and makes it almost stop cranking. I used the 7X external crank trigger along with the factory 24X sensor. The GM shop manual says the 7X sensor should put out 2 volts, but I've tried a couple of them and get between .4 and .5 volts.

Likewise, there has been some discussion here about the sensor leads needing to be twisted at about 9 turns per foot. I got a section of the sensor harness from a 1999 Malibu with the quad four and the wiring was dead straight and not twisted. But my harness is twisted.

Fuel pressure is 41 psi and my Autotap software says there have been no DTCs set. The only thing I see wrong in the data is that the cranking speed jumps from about 200 to about 700 at random. The aftermarket 7X sensor package lets you adjust the timing, which I have done as far as it will move. If I advance it too much, it cranks poorly, too retarded and it spins easily, but it still keeps getting some pulses too far out of time. I've checked the plug wires again and again and they are correct.

So, I have fuel and I have spark, but it sure seems like the spark is not coming at the right time. Any suggestions?

[IsaacHayes]

Sure on the plug wires being in the right spot???

[kerno]

Yeah, they are. If 246 are on the front of the engine and 135 are on the rear, they are right. I even tried advancing them and retarding them by one step. It makes the problem worse. The key seems to be the RPM signal. The datalog shows it jumping around - a lot. In one cranking cycle, the log picked up 6 data points, where the speed went 190, 59, 565, 205, 684 and 53 RPM. The cranking speed is not changing that much, so it seems to be cranking speed signal that is a problem. According to the way I read the FS Manual, the engine uses the 24X during cranking and all that stuff is stock. But it has to use the 7X sensor to find TDC. Likewise, if there is a conflict in the data from the 2 sensors, it should set a trouble code. I can pull the balancer and make sure everything is straight and square, but the 3100 ran fine until it broke a cam, so all the sensors and electronics "should" be fine.

It is possible that the strange cranking speed signals are from the engine trying to run, which gives the 600 or so RPM signal and then it trying to run backward when the pulse is too far advanced creating the 59 and 53 RPM points.

[2000GAGT]

I swapped a 3500 into my grand am gt last spring. I would leave the 7x sensor timing at 0. I tried adjusting mine and it made the car stumble on acceleration. my fuel pressure is around 48psi at idle with my old 22.5lb injectors from the 3400.

Did you check for vacuum leaks? everything worked perfect for me on the first start up except for a nasty leak coming from the o-rings on the injectors into the LIM, i had to swap for thicker rings.

You can disconnect the 24x crank sensor and attempt to start the car, if anything changed that could be your answer because the car should start and run without it connected.

If it could be spark check the ignition control module, mine burned out after 6k miles of driving, i brought it in to an auto store to be tested. the car would start and run but randomly stumble and stall if you didnt give it gas

[IsaacHayes]

Yes, odd on fire wall, even by radiator. Don't get confused by the LIM numbering as some people have, that just points to where the runner ends up at.

Are your CPS wires not twisted? If molested any at all they can be problematic, they are very susceptible to interference so they need to be twisted to cancel noise and cross talk. Think of network cable with twisted pairs..

[kerno]

I rerouted them earlier so they were completely away from the plug wires and they are twisted. In hopes that it was old fuel causing the problem, I tossed a new set of plugs in it, drained the fuel tank and put in 5 gallons of fresh fuel.

Same result. It really wants to catch, but won't. As I go back over the data logs from trying to start it, the MAP and the MAF both change as I move the throttle during crank. Likewise, the timing changes.

My most recent crank showed this:
545 RPM, timing 25 degrees, MAP 28.3, MAF 1.20#, Throttle position 10%
323 RPM, timing 21 degrees, MAP 29.5, MAF 1.16#, Throttle position 20%
244 RPM, timing 18 degrees, MAP 29.2, MAF 1.85#, Throttle position 29%
204 RPM, timing 18 degrees, MAP 30.4, MAF 2.70#, Throttle position 64%
567 RPM, timing 15 degrees, MAP 30.4, MAF 1.56#, Throttle position 72%
326 RPM, timing 12 degrees, MAP 30.4, MAF 0.80#, Throttle position 72%
502 RPM, timing 17 degrees, MAP 30.4, MAF 0.88#, Throttle position 72%
502 RPM, timing 17 degrees, MAP 30.4, MAF 0.86#, Throttle position 72%
253 RPM, timing 17 degrees, MAP 30.4, MAF 0.79#, Throttle position 69%
201 RPM, timing 17 degrees, MAP 30.7, MAF 0.75#, Throttle position 22%

It must be seeing the throttle and rpm change to vary the timing. It must know where TDC is and when it occurs. I double checked to make sure the pin in the reluctor wheel lined up with the crank key and even yanked a valve cover to be sure the intake and exhaust were both closed at TDC for # 4. I used #4 because 1 is hard to see! The valve event appears normal.

[SappySE107]

why is there throttle position? That should be 0.

[Weatheralls Auto]

....the MAP and the MAF both change as I move the throttle during crank. Likewise, the timing changes....

why is there throttle position? That should be 0.

Besure you orientated the wires correctly, and have the 7X @ 0*
It is the 7X that is used for starting, not the 24X

[2000GAGT]

My throttle position is always between 0.2-0.4% when closed

[kerno]

The throttle position was there to demonstrate that most of the functions seemed to be properly interfaced with each other. Most of my starting attempts have been by reaching in the window. It does not seem to make any difference at all where the throttle position is.

Today's effort is going to include marking the reluctor with the firing order and using a timing light on the individual wires to see which plug is firing when - if that is possible. It continues to sound as if I have the wires switched, but I don't think I do.

Here's the order I am using, which is exactly like the illustration on page 6-1859 FSM for the L/N chassis. In my picture, you can see the original 3500 map sensor still in the UIM. It is not hooked up because I am using the stock 3100 MAP sensor.

[bszopi]

Have you checked the gap for the 7x sensor to the reluctor? If the sensor is too far away from the reluctor, it won't pick up a proper signal. It needs to be as close as possible to the reluctor.

[kerno]

How close should it be? I spaced it out from the block a bit to center the pickup over the wheel and away from the un- notched balancer. The gap is currently about .030, but I'll go close it up and let you know if it helps. That external setup has a lot of nice machine work in it for the price!

[bszopi]

0.030" should be good. Have you looked for any other electrical issues, ie battery charge or loose grounds? That would be the next thing I would start to look at. If you've been cranking it a lot, you are draining the battery. If the block and/or battery isn't properly ground to the chassis, then you may have issue with getting complete spark.

Have you looked at the plugs to see if you have fouled them? Or if the cylinders are flooding?

[kerno]

I got the gap down to .012 and there is no change. When I swapped the plugs yesterday, the ones that came out were wetter than they should have been. None of them were dry and clean which tells me the mis-fire is affecting all the cylinders. The battery is fresh and the charger is is nearby.

I agree that grounds are a frequent problem, so I'll go back over them. The spark appears to be complete and strong enough to make it want to fire. It has that telltale problem of cranking smoothly for several revolutions until a spark is fired that it so early that tries to make the engine run backward because it occurs too far before the piston is at TDC for the starter to overcome it without hesitation..

[2000GAGT]

Did you check the crank sensor power wire at the ignition module to make sure it's not fried and the signal is reaching the coils