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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
My 1991 305 fuel injected Trans Am has a severe surge starting at about 1500 rpm, up to about 2000 rpm, at low, steady throttle. This is so strong that it broke a transmission mount (now fixed). Above this rpm, engine runs very well. Below this rpm, engine also runs well. If I get after it, it goes right through this rpm range.
I can't complete a California smog test because there is a test segment exactly in this rpm band, so I can't even get the car registered while this problem exists.
ANY suggestions about this problem cause and how to fix it would be very much appreciated !!!
Car - 1991 Firebird TA, 1G2FW23F3ML202439Engine - 5.0 TP injection
Bosch injectors, installed years ago, otherwise all stock power train.
Transmission - 5 speed manual
Owned since new.
BTW - Photo was taken to convince LA County to fix driveway, now done...
Last edited by Roger Davis; 06-04-2023 at 01:40 AM.
Reason: additional info
What "Bosch injectors"? They make about a million different ones.
Has it always done this? or did it run fine at one time and now it's like this?
What's the fuel pressure WHILE IT'S IN THE ACT OF SCREWING UP? That reading needs to be taken WHILE IT'S IN THE ACT OF SCREWING UP; NOT while it's sitting in the driveway idling happily; WHILE IT'S IN THE ACT OF SCREWING UP. Meaning, tape your FP gauge to your windshield, take it out for a drive, and have your assistant watch the gauge to see what it reads WHILE IT'S IN THE ACT OF SCREWING UP.
Did I forget to mention, that the ONLY PERTINENT THING is, what the fuel pressure is WHILE IT'S IN THE ACT OF SCREWING UP, as opposed to sitting in the driveway idling happily? If I forgot, let me be clear, we need to know what the fuel pressure is WHILE IT'S IN THE ACT OF SCREWING UP. Whatever it reads while sitting in the driveway idling happily DOES NOT MATTER and DOES NOT MEAN DIDDLY and is a WASTE OF TIOME AND INTERNET BANDWIDTH.
Any questions about what you need to measure? Does any letter of the words WHILE IT'S IN THE ACT OF SCREWING UP need further clarification?
The injector data is at a different location, but they were purchased as matched to this car and engine, and have been in place and running well for several years. Doesn't mean one of them hasn't gone bad... However, my feeling is that the surge is so severe that it would likely take more than one of them acting up.
Haven't yet measured the fuel pressure, and will follow this up. Question - can the pressure be bad at only this narrow rpm range? Every other rpm, above and below, is great. The fuel pump was replaced a few months ago, which makes it a bit suspect to me.
I don't have in hand the logged engine data or codes, but will get it shortly (probably Monday). Without specific data, was told "Engine is working fine" from the repair shop taking care of it for years, I have to believe it's been checked, but I need to make sure.
Any ideas about a control mechanism, mechanical or electronic, that does a transition or function at this rpm range that could be screwing up?
Last edited by Roger Davis; 06-04-2023 at 01:49 AM.
Reason: additional info
The injector data is at a different location, but they were purchased as matched to this car and engine, and have been in place and running well for several years
"Injector data at different location" and "purchased" and "matched" isn't real useful. What part # is printed on them? What color are they? Got a photo? We can't help you without you giving us information. YOU gotta tell us; it's too dark right now for me to look through your monitor under your hood and see what they are.
Although, it's useful to know, it hasn't ALWAYS been this way. At one time evidently , whatever injectors they are, they must have worked better than whatever is going on now. Note that I am NOT saying "your injectors are bad"; NOT saying "replace your injectors"; NOT saying "put your stock injectors back in"; NOT NOT NOT NOT NOT NOT NOT NOT. All I'm saying is, you just told us that it used to work OK.
Butt plainly SOMETHING has changed since then. That makes it MUCH eeeeeeeeeeezyer: all ya gotta do is, figure out what's changed. Can't be all that hard, really.
Haven't yet measured the fuel pressure
Well, get to it. Tuned's suggestion about data logging is AWESOME, except that it requires buying all kinds of stuff, building other stuff, and in the end, you're STILL stuck with the data rate available from that 70s computer-like thing... which was around 90 baud or so if memory serves. You get something around one full frame of data per second, maybe less; not real good for something that comes and goes. For a condition that's "transient", as opposed to "continuous" or "sustained", the data stream you can get from it is ... ambiguous, at best. While a great tool in its own right, and immensely useful for many things, it's not the one, out of all that are in my toolbox, that I'd reach for first, in this kind of situation. I'd get more BASIC first.
can the pressure be bad at only this narrow rpm range?
Yes and no. I doubt that the FP is falling down in some way in this specific narrow range. However, low FP can exhibit more, or less, severe symptoms at ANY RPM range o runder ANY random set of circumstances, depending on whatever else is going on at the moment. It's the ABSOLUTE FIRST THING TO LOOK AT whenever a EFI system is behaving erratically. Even before datalogging.
shop
What is THAT? I've heard of it before, mostly from women who are buying more shoes or handbags or some other sort of woman stuff; not sure how that relates to a CAR? Please explain.
Any ideas about a control mechanism, mechanical or electronic, that does a transition or function at this rpm range that could be screwing up?
No. Only that whatever defect or inadequacy is occurring, has a more pronounced effect on your engine, and YOUR BRAIN (bozhye I wish I had one of those) in that specific operating window, than in any others that you've noticed. Especially since the Emissions Police are watching you.
Yes I've lived in So Cal, though I have since moved on. (I move alot) Yes I know EXACTLY what you're going through. I was in Carlsbad right where Carlsbad Vista & Oceanside come together.: I moved there from Tennessee with my 83 L69 car which had a 400 with a kinda big solid cam in it at that moment. So yeah, I feel your pain, I know what it's like to have to deal with those people. And I can assure you, blaming "shop" and whatever all else for YOUR problems, might make you feel a little better for the moment; but won't get you ONE DAMN BIT closer to solving your problem. YOU have an ANTIQUE car, which is quite a bit different from a ... more current model. Paying "shop" to indulge in your ANTIQUE car hobby FOR YOU is one of the stuuuuupidest ways I can think of to enjoy an old car. You gotta step up and learn to take care of it YOURSELF, otherwise you're gonna be BLED DRY, and NEVER have a car you can drive no matter how much blood you let em drink. They'll just keep comng back for more, doesn't matter if you start turning grey or even bleached white.
So, [rant mode off] let's move directly on to the VERY BEGINNING, before even Step 1, aka Step 0: what's your fuel pressure WHILE IT'S IN THE ACT OF SCREWING UP? Don't argue about "can it do it"; just MEASURE IT.
R.E. Sofakingdom
- I'm away from home until Tuesday, all my information is at home. When the mechanical requirements exceeded my work area and tool capability, I took it to it's "other home" the auto shop near my house that has been doing all the major work on it for many years. I'll need to get some of the info, such as fuel pressure, from them.
- I did make some changes to my post for clarification - "... severe surge starting at about 1500 rpm, up to about 2000 rpm, at low, steady throttle." and "If I get after it, it goes right through this rpm range." Does this info give any further ideas?
- The problem is transient only if I continue to accelerate. Hold the throttle steady, it keeps it up until the revs go below 1500 or above 2000, or "it throws you out of the seat". Stand on it from the start, you probably wouldn't even notice it.
- I've gotten lots of info here, most importantly about fuel pressure. Excuse the scorch smell, I'm thinking... pump is electric, so absent a weird electrical problem at the problem rpm range, fuel pressure "should" be higher at low fuel usage (problem rpm range), and lower as more fuel is used? I don't know how an electrical problem of this kind could happen, but maybe check system voltage and current in tandem with the fuel pressure ? Or am I way off?
- Is the pump an impeller type, piston type, diaphragm, or ??? I've got no idea, but would guess diaphragm.
The pump is an impeller type. It's constant-volume; it's supposed to deliver a volume to the engine far in excess of what it will ever use, and the pressure regulator returns enough of it back to the tank, to hold the pressure at the desired level. If there was no return, like if the line was blocked, the pressure could go as high as double what it should be, maybe more. In these cars, it's designed to run at about 43 - 45 psi with no engine vacuum (throttle wide open), down to around 36 - 38 psi at idle and other very low load situations. The regulator is connected to manifold vacuum to give it the reference for setting the correct pressure.
I don't have much confidence in "shop" being able to accomplish much of anything in this matter besides paying for their kids' braces and college funds off of your bank account. You're wasting your time and money that way. Go to the parts store and buy a fuel pressure gauge, hook it up, and do the test. "Shop" will charge you AT LEAST twice what a gauge costs JUST TO DO THAT, and then more besides to "fix" whatever is wrong with it. This is a forum where you can find out how to DO IT YOURSELF, not find out anything about "shop".
I'm strongly suspecting a fuel delivery type issue of some sort. Cause is almost certainly the pump, regulator, or injectors. Your goal is to narrow that down to ONE possibility. You don't need "shop" for that, just a FP gauge. Unfortunately, in the case of fuel pumps, "new" is no guarantee of "right". You don't say, for example, why you changed the pump, what it was doing that brought you to that point, whether it started this other thing before or after that change, how long it's been doing this, whether anything else has been done to the car, and so on. In addition to gathering the data, applying some logic based on the surrounding history might turn out to be useful as well.
As regards "shop", and do it myself - I'm just short of 78 years old, and just not capable of what I used to could do. At one time I was employed by one of the major powersport vehicle manufacturers (18 years), and traveled the US in support of all vehicles, from golf cars to racing snowmobiles, and worked with dealers, factory race (two years as a race team tech) and support drivers, including Dick Trickle (see below). I did another 22 years with a second OEM, but less mechanical there, as I was in charge of accessories. I've lost only a little of the ability to think things through, but other abilities have not fared as well, though I still know which way a wrench turns. The shop I speak of has always treated me well, and been competent, though I sense that they could use more information on this specific case, hence my research. BTW, I bought the TA direct from PMD Zone office, ordered to my spec. I've got everything from the build sheet down to the window sticker.
I agree with the fuel system suspicion, as after you hold the problem rpm for a while, it gets so severe that I could believe most or all power is going away momentarily, returning, then going away again. "Almost" surely more than one injector... The original pump lasted until just a few months ago, when it was replaced. It's still soon enough that a flaw could just now be showing up.
I agree with the fuel system suspicion, it just seems odd to me that it's limited to such a narrow specific rpm and throttle opening range - makes me think about some kind of problem in a control system transition point or another transition. Or Engine computer?
Your information allowed me to consider this - Is the fuel pressure regulator integral with the pump, or a separate item? If separate, where is it mounted? One idea would be that the regulator is screwing up at higher vacuum, and delivering too little fuel (high vacuum due to low throttle opening and higher than idle rpm needing more than idle range fuel), and popping up to higher volume when throttle is more open. Where is the vacuum line that controls the regulator?
Since I don't see anyone else talking about it... Take a look at the ICM... My 88 IROC had an issue with bad misfire in this same range when under light load .. changed ICM... Totally disappeared!... If the timing is floating, it can surge the engine...
Just throwing it out there as it may not be fuel related... But ignition
Hang in there Roger, I agree with the getting older and not being able to do exactly what you want with tools. The tools we have used all our lives. I get pissed at my body not functioning 100%
Fuel pressure regulator is on one of the fuel rails, at the rear.
ICM is a definite possibility. What's more, not too $$$$, and eeeezy to get to. If fuel related things all seem to be OK, you could try that.
I can't imagine that it would be the ECM, but I won't say that it's not, either. They are obviously in charge of everything and if they screw up everything else is doomed. I just wouldn't jump to that sort of conclusion without ruling out the more likely things first.
Without sensor data , integrator and block learn numbers it’s a total crap shoot to fix.
sounds like a lean surge. A adjustable fuel pressure regulator might help cure the problem. Did the issue start with injector replacement?
No, the injectors have been installed for several years, working fine, the surge problem just started.
Fuel Injector info (requested by sofakingdom ) - Bosch 3, Yellow, 22 lbs, with adapter ring, can't switch adapter, use double o ring if no adapter. Bosch or Standard Brand number 0280155700 or 0280155710, from South Bay Fuel Injectors
The injectors are faded yellow, too light to have ever been orange.
After the trans mount was replaced, it was smooth enough to just get by the low rpm smog test, but not perfect.
Now that I can drive it and check it out more, it seems that the engine is fairly smooth in the 1500 ~ 2000 rpm range in neutral, or driving with load on it. It does surge and jerk when driving in almost no load, or very slightly downhill.
So now it seems like the drive train has enough flex to "wind up and unwind" and multiply the motion and slack in the drive train (manual 5 speed). What starts off as barely noticeable quickly grows until it's much stronger.
Does anyone have any thoughts on this?
Maybe someone out there with a manual transmission in a pretty much stock third gen car can try this: second gear, very slight downhill, hold steady at between 1500 and 2000 rpm, and see if you get a similar jerking action, and please let us know.
The injectors are faded yellow, too light to have ever been orange.
After the trans mount was replaced, it was smooth enough to just get by the low rpm smog test, but not perfect.
Now that I can drive it and check it out more, it seems that the engine is fairly smooth in the 1500 ~ 2000 rpm range in neutral, or driving with load on it. It does surge and jerk when driving in almost no load, or very slightly downhill.
So now it seems like the drive train has enough flex to "wind up and unwind" and multiply the motion and slack in the drive train (manual 5 speed). What starts off as barely noticeable quickly grows until it's much stronger.
Does anyone have any thoughts on this?
Maybe someone out there with a manual transmission in a pretty much stock third gen car can try this: second gear, very slight downhill, hold steady at between 1500 and 2000 rpm, and see if you get a similar jerking action, and please let us know.
"erik69&85
Since I don't see anyone else talking about it... Take a look at the ICM... My 88 IROC had an issue with bad misfire in this same range when under light load .. changed ICM... Totally disappeared!... If the timing is floating, it can surge the engine...
Just throwing it out there as it may not be fuel related... But ignition
06-04-2023 07:00 PM"
I'd be looking there as well. Same thing happened to me. Is it worse on a warm engine?
I am having a similar issue with mine while cruising at part throttle. I thought it was a miss, so I got all new plugs, wires, looms, boot protectors (bc of the headers) and it still does it. I swapped the engine several years ago with bigger cam, valves, injectors (Bosch blue ones from South Bay), Stealth Ram and it's done it since. I have to pay attention to the FP when it does that. I don't remember what it does WHILE SCREWING UP. Is it possible it could be something in the drive line, other than engine related. I swapped to a T-56 and am wondering if it could be clutch related? But I'm thinking I'd be getting more of a chatter or vibration than a surging if that were the case.
Will FP drop under load? I'm pretty sure I have the factory pump. I know I didn't change it.
My problem is at barely open throttle and very light load. It feels like someone shutting the ignition off and on at about 1 ~ 2 second intervals. The only similarity I see is the Bosch injectors from South Bay. I might give them a call...
Last edited by Roger Davis; 06-28-2023 at 10:40 PM.
Reason: accuracy
You sure it’s not a miss from a bad plug wire? I took extra precautions to keep my wires away from headers because I had a couple melt. One was even arcing thru the insulation. I just happened to notice it.
Plug wires replaced recently, and rechecked now. Also, if plug wires were causing it, there would be no interval between surges and problem would occur at higher loads and speeds as well.
06-03-2023, 04:49 PM
I'd be looking there as well. Same thing happened to me. Is it worse on a warm engine? 
