c44ever

My '90 L98 (auto) accelerates fine but idles rough and when driving at a constant speed, it has a surge or "bucking" problem. I ran a Datamaster scan on a 10 mile trip at 0-45 mph. The long term counts stayed right at 108 with a few increases to 114. The spark advance varied sharply from 35 to 15 and at idle the spark advance jumps between 15 and 22. The knock count only got to 18. Could the injectors be causing the spark advance to vary so much or is something else ??

mseven

35-15 varrying would need a better description of when it is occourring. However, it might be close throttle position at a higher rpm/ or mph, and depending where the table is set-up (timing settings for "closed throttle vs. rpm") might be what you are seeing (also depends on if the these or any tables have been modded). The bounce at idle is somewhat normal but can be tightened up with accuarte fuel/timing tables, distributer end play etc.
108 is pig rich and the first place I would be working on. Injecters would contribute to 108 BLM, (very rich) which is out of the control of what the ecm can comp.

Sam Lam

Just for chits and grins, check the resistance of your injectors. Easy and simple test. 16 to 17 ohms when cold and all should be very close to each other. Check 'em when hot and the reisitance should be higher than cold, but very close to the same number. A BLM of 108 is very rich

c44ever

Okay, I checked the resistance on the injectors and the following is what I got:

COLD
1 = 10.7
2 = 16.6
3 = 16.8
4 = 6.9
5 = 10.7
6 = 16.6
7 = 15.4
8 = 11.1

HOT
1 = 11.5
2 = 17.2
3 = 17.4
4 = 7.0
5 = 11.2
6 = 17.4
7 = 16.0
8 = 11.5

Looks like I got about 4 bad ones don't I?
Could these be causing the extreme rich BLM and the roughness?

Thanks,
Roger
'90 C4 L98 Automatic Coupe White/ glass top.

Sam Lam

Some comments for you:
1. As the turns inside the injector coil began to short out, the resistance drops since you have lost "effective" turns in the coil.
2. As the resistance drops (lose effective turns of wire), that injector will introduce LESS fuel into that cylinder. This causes a lean mixture.
3. The O2 sensor is located in the exhaust of cylinders 1, 3, 5 and 7 (the data from the drivers side of the engine is also used the adjust the injector pulse time on the passenger side injectors). You have two injectors on the drivers side of the engine that is supplying less fuel than expected and you have an "overall" lean mixture. The O2 data indicates a lean mixture and the ECM attempts to add fuel....and it cannot introduce more fuel due to shorted turns in the injectors. The BLM value goes to the limit of 108 or pig rich and you are at the limit of the controls designed into the closed loop system.
4. As for running rough, all eight injectors have to introduce equal quantities of fuel to each cylinder (recall that you should have a matched set or equal fuel delivery rate when you buy new injectors) in order to have a smooth idle. You will not have this condition with the injectors currently in the engine.

I am not an expert on this and still learning how this simple system can be so complicated. Hope the above is correct as it is my opinion from reading info on fuel injection.

c44ever

Okay thanks. I ordered a new set of the Ford 22lb injectors. They will be shipped today so I may have them in time to install this weekend. After install, I will run another scan and see what, if any, effect it will have on the BLM.

c44ever

The timing jump occurs at anytime regardless of throttle position. I was on a straight level stretch of of highway and kept the speed at a constant 45 mph and the timing would drop from 35 to 15 for a few seconds and then jump back to 32-35. Thats when I could feel the surging.

mseven

is this car stock? your first post shows a rich condition and at 108 blm which is beyond the ecm's control, so if not stock tuning is needed. if stock, there is something else here that would cause a rich condition,(fueling, 02 sensor, leaking inj. etc,etc.)
based on your comments... holding throttle position should not cause timing drops (closed throttle will).
Regardless, it sounds like an electrical (wiring from dist. pinched, worn etc., grounds etc.), or mechanical issue than in the tune. I would also examine dist. (gear etc.) ....est. is connected, right.

c44ever

Yes, the car is completly stock and all connections are made. From the above posts, it looks as though I do have some bad injectors, a new set should be delivered today so that I can get them installed and run another scan. I will also start checking and cleaning all connections.

Thanks!

Sam Lam

c44ever: I purchased my 89 car in 2001 with 39k miles on it. When driving the car about 40 - 50 MPH on a level ground, the car would surge or "buck". If I were on an uphill grade, the car would not surge. I did not have a scan tool and could not see what was happening with the engine sensors. After two years of looking (and losing more hair), I finally realized the injectors were shorting out. I got rid of the Multec injectors and the surge problem has not been seen since this change. I cannot explain the connection between the shorted Multecs and the surge issue.

I have a procedure to change out the injectos without pulling the runners. Send me an email if you want a copy of this procedure.

FYI only.

c44ever

Installed the new injectors last night (the hard way) and the car runs better than ever. The bucking is gone, idle is smooth, acceleration is great. Ran a scan just to see what changed. The erratic timing is gone, it varys only with acceleration/rpm. However, the BLM is still pretty much pinned on 108 with a few counts in the 116-118 range then right back to 108. I watched the TPS and it is .69 voly at idle and 1.52 at WOT. Is this acceptable ??? what else could be causing the super rich condition and what can I do ? The air to fuel ratio stayed at 14.73 for the entire scan.

mseven

a lazy 02 can cause a rich condition, and might be a good place to start.

Sam Lam

c44ever: I thought you had some bad injectors I am not familiar with the 90 TPS......but I am very familiar with the 89. The TPS setting on the 89 is .54 +/-.06 volts at idle as this sensor is adjustable. At WOT, I get about 4.3 to 4.4 volts. You should be somewhere in this range with your 90 model.

When you did the scan, did the O2 reading bounce from .1 to .9 volts in a rapid manner? in addition, did you check the fuel pressure? It appears the ECM is controlling the air to fuel ratio properly. However, it working on one extreme side of the tables to do this.

c44ever

The O2 volts jump all over the place, fills the screen on the chart. Sounds like maybe I need to check the fuel pressure ?

larryfs

I heard the opposite. The injector current will increase, and the coil will saturate with current eariler, and longer, therefore dumping more fuel. Then depending which side it's on, yada, yada....

Sam Lam

The reading of the TPS of 1.52 volts at WOT is not normal for my 89. Suggest you hook up your scanner, turn on the ignition key (do not start engine) and manually operate the butterfly valve on the throttle body to a WOT position. When the butterfly valve is wide open, note the TPS reading on the scan tool. It should be above 4.0 volts on my 89 Vette.

mseven

info above remains the same is for 90, at wot 4.5-5.0, exception being is that closed throttle position for tps is fixed, and should read anywhere from .6-.69.

as long as it is red

Is non adjustable. There is a little tab on the TB for the TPS alignment. All I can do is hook the TB shaft lever thing to the TPS arm. I guess you could elongate the mounting holes on the TPS to clock it where you want it to get the desired volt reading.

Sam Lam

If you cannot get more than 4 volts on the TPS with the gas pedal to the floor you have one of two possible problems:

1. The TPS is defective and needs replacement
2. The cable linkage to the butterfly valve on the throttle body is binding or not working properly.

You can manually move the butterfly valve to WOT (engine not running and ignition in the ON position) and note the TPS voltage on the scan tool. This will indicte which of the two problems you are experiencing.

Let us know how this turns out

c44ever

I ran the TPS sensor all the way to WOT and still only got 1.52 volts. I replaced the TPS with an adjustable one and it still only goes to 1.52. That is pushing the tab on the TPS all the way to WOT.

We just drove the car 1500 miles this past weekend to Cruisin' The Coast in Biloxi and it ran like a dream getting 28 mpg !!! The scan still shows BLM of 108 after warm-up.....