88 2.5 Timing (Page 1/1)
shawn95 MAY 31, 08:52 PM
Hello everyone, I'm new to the forum and to Fieros in general so i apologize if this question is already answered somewhere (I tried to do a bunch of research first before making a post).

Little background: My uncle's 88 Fiero lost it's clutch/TOB and he put it away in a barn with plans of fixing it. Fast forward 25 or so years to October 2021, and he finally agreed to sell it to me. Even though it was sitting for so long, I am happy to say I haven't found any chewed wires or anything and no major issues other than some rusty parts and crusty bushings (as expected).

Now onto the issue at hand. I've been tearing down the engine and long story short I want to make sure the timing will be right when I put it all back together. I did take the cam out to press off the old fiber gear and press on a nice new aluminum one, but I'm not 100% sure I have it back in the same exact position it was in before. I put the lifters and pushrods in and turned the crank to make sure the correct valves are opening and closing at the right time. It seems like they are, but again I'm new to this so I'm not 100% sure...

I did some searching around and saw a similar thread that said you cannot adjust the timing in the 87-88 2.5. Which is all well and good, but my question is how do I know everything is in the right position to begin with? When I was turning the crank as mentioned above, I watched the crank notches that the sensor reads and noticed there's several single notches and then 2 right next to each other. This "double" notch passes the sensor hole just after where piston 1 would fire and just before the exhaust valve starts opening. I just don't want to get the engine all put back together, hook it back up to the car and fire it up and have it be all wrong.

Thanks in advance for any replies and help!
theogre MAY 31, 09:04 PM
This engine has more changes than DIS...

See my Cave, Duke Quick Ref, DIS Ignition, Ground "Myth" notes & 700 TBI while there.

This DIS version Needs the shield shown there or will have electrical problems or worse.
And ECM is programed to have EGR working or will have problems sim to engine miss or ignition break up when driving even tho EGR is "dumb."

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

shawn95 JUN 01, 06:34 PM
Thanks for the reply Ogre,

I went and checked my DIS and the plate is there so that's good. I also have a new EGR valve and crank position sensor ready to be installed.

I was reading up on the links you sent to your cave, while I was reading through the "DIS Ignition" page I saw this:
quote
The only moving part in DIS is the crankshaft notch wheel. The crank position sensor generates pulses as the cuts in the notch wheel pass by it.

The ECM and PIM control Spark timing based on pulses from the crank position sensor and a number of other factors. (Proper operation of the MAP sensor is critical. The ECM uses the MAP sensor to calculate Vacuum Advance for the spark amongst other things.)

Coil selection is done by the PIM based on the input from the crank position sensor.



How do those notches on the crankshaft notch wheel communicate which piston needs to fire? Does that double notch correspond with any of the pistons?
theogre JUN 02, 12:08 AM
Two notch section on the crank is "home" to start coil 1/4. See https://www.megamanual.com/ms2/GM_DIS.htm etc.

While cranking, DIS mod is in control of Timing. ECM takes control after say 500-600RPM.

Many times DIS Mod and ECM takes 2 to 4 or even more complete revs to start ignition and fuel injector.
Often if you "bump" the key to start and doesn't start quick, then stop but don't turn key off, then start again is faster then 1 long crank time to start.
shawn95 JUN 02, 09:21 AM
So much good info in that manual, thanks for that. I found this in the GM DIS section talking about those two notches in the crank,
quote
In a 4-cylinder application, the notches DO NOT coincide with TDC, except for the one at TDC for cylinder#1. The sync pulse comes at 70° after TDC for cylinder #1.



Reading that, I think I have it together correctly. Back before I took the cam out I marked where the cam gear and crank gear lined up, but the crank got turned somewhere along the way while the cam was out and that's where my uncertainty came from. That manual says the sync pulse happens after TDC for piston 1. So seeing the sync pulses shortly after piston 1 fires at TDC on my engine seems to line up with what the manual is saying.
theogre JUN 02, 01:39 PM
The Cam has Nothing to do w/ Ignition Timing for DIS.

The DISK for Ignition timing is Part of the Crank Shaft. Some are cast when crank is made like Dukes is. Others are bolted on often part of Front balancer/pulley assembly.

Some L4 V6 and V8 uses a Cam Sensor because they have Full Port Fuel Injection and need to know both Cam and Crank is on #1 to setup EFI Ports. TBI and Bank Injection don't use this.

Duke Cam Gears have Keys and Timing Marks to install them. And if Marks are off then won't run right or not start because valves timing even tho has nothing w/ ignition timing.
shawn95 JUN 02, 02:00 PM
Right so since the crank wasn't messed with, ignition timing shouldn't be an issue. Its just making sure I have to valve timing correct.

Thanks for all the info.